Assertiveness - should I have feeling that someone need to do something for me I have seen a assertiveness training videos and there is said that the person should show assertiveness with "I" statements.

For example :
I feel seed due to the way you just spoke to me.

My problem is that I see that this can't be used, as in my opinion it is just a manifestation of weakness. If there we split all possible situations in just 3 :


in family or with love one
at work
on the street


The only place where this type of place where this type of sentence would not be seen as a weakness is maybe only in first case , so the question is, why is this so ? Am I wrong or the world is wrong ?

Second example :
I feel that you don't invest enough of time in keeping the apartment clean.

In this type of assertive sentence, I see the problem that someone is saying what they feel or what they think but the only way to fix that is that other person do something about it. This one way view of this is also a major brake in using sentence like this one. The second problem is that if somone would use this type of sentence offten, he would be a needy person ? And also here is the same question. Am I wrong or the world is wrong ?
From what I could understand, you are broadly asking:


How can I ask for something assertively without seeming demanding or weak?
Is it ok for me to be demanding and even selfish?
How am I expected to be assertive under different circumstances / environments (e.g work, family, general social interactions etc)?


It helps to think of "Assertiveness Training (AT)" as:


a way to effectively communicate with others 
considering the rights of all party involved in a social interaction, and 
keeping in mind the context of the social interaction.


Just like a constitution in a democracy gives you certain "rights", AT also defines some rights that you need to recognize and understand to be assertive in a social situation. 

These rights are: 


Every individual has the right to do anything as long as it does not hurt others.
Everyone has the right to maintain their dignity by being assertive - even if it hurts someone else's feeling.
Everyone has the right to make a request from someone, as long as they recognize that the other person has the rights to say no.
In many interpersonal situations the rights aren't clear. But you always have the right to discuss the problem with the persons involved, to clarify it.
Everyone has the rights to these rights.


Source: Fensterheim Ph.D., Herbert and Jean Baer, (1975). Don't Say Yes When You Want to Say No: Making Life Right When It Feels All Wrong. RHUS. 

(While these are broad guidelines, there are others who have expanded it to be more specific; e.g here and here (pdf)).

We have all come across people who are aggressive or passive.

In the context of AT:


An aggressive person is someone who doesn't respect the rights of others nor cares for their feelings. Aggression is about dominance. E.g. of this kind of behaviour include yelling at someone, threatening someone or even physical brawls to get your way.
A passive person on the other hand is someone who sacrifices their own rights or feelings and often gives into the demands made by someone, irrespective of what they genuinely want to do or how they feel about it. Passivity here is about being submissive. E.g. of this kind of behaviour include doing what everyone asks you to do, trying to be nice to everyone even if you don't feel like it, not standing up for your beliefs etc.


So assertiveness is kind of like trying to navigate another path between being aggressive and passive by being considerate of the feelings / rights of others you are interacting with while also respecting your own feelings / rights too depending on the situation and the person(s) involved. 

Thus, an assertive approach is about respect and consideration for self and others. 

Under certain circumstances, deliberately choosing to be aggressive or passive can also be considered being assertive. 

Another important concept to understand in AT is to understand the situational context of any social interaction. 

That means, depending on the situation, context and the person(s) involved, how you communicate also needs to change. 

E.g. When your wife yells at you, you may feel comfortable to yell back at her. But obviously when your boss yells at you, being assertive-passive and choosing to be quite is a better choice at that moment. 

Or, for example, it is not assertiveness if you express your undying attraction and love to a widow at her husband's funeral, however badly you want to, because it is socially not appropriate.

So now, let's address your doubts:


How can I express or ask for something assertively without seeming demanding or weak?


Keeping AT concepts in mind, being demanding can be an aggressive behaviour if you are totally disregarding the rights of the other person (i.e. their right to say no to you). So if you are not disregarding the rights of the other person, as per AT you are not being demanding.

But you also have to consider the situational context. E.g. Sometimes you may choose to be rude with a stranger if they are annoying you or you may even be aggressive with someone if threatened by them. Or you might purposefully choose to obey and be passive when you feel threatened, so as to not endanger your life. 

As a boss you can make some demands on your employees to do their job better or in a particular way. This may seem like an aggressive behaviour, but can be considered an assertive approach because of the social context - a boss is required to manage his employee, and employees are expected to do their job well. 

About appearing weak - AT does not consider expressing your thoughts, wants and feelings as weakness. In fact it actually encourages you to express it clearly, directly and openly. (e.g. "I didn't like that movie", "I want to go a movie", "I felt bad when you arrived late for the movie"). Expressing yourself like these isn't a weakness because it helps the other person(s) understand you better, without any room for misunderstanding. 

Ofcourse, most of the times, it is not assertiveness if you aren't considerate of the other person's rights.


Is it ok for me to be demanding and even selfish?


Depending on the situational context, yes. 

You have the right to say no, even to your close friend, even if they may feel hurt about it. 

Obviously, assertiveness doesn't mean you always have your way. But AT says you should also be considerate to yourself too (you have the same rights as others), and so if you feel someone is making some unjustified demand on you, it is ok to say no even if some may perceive that as selfishness.

You can be demanding if required so. For example, at work, sometimes you may need to be aggressive to further your career. (Note that the appropriateness of this varies based on a society's culture. For e.g. in US, it may be appropriate for you to go home before your boss, but in Japan or Korea, this may be considered as you being rude to the boss). 

(It is very important to keep in mind that AT is not about always getting your way). 


How am I expected to be assertive under different circumstances / environments (e.g work, family, general social interactions etc)?


By balancing your rights vs the rights of the other person(s) in a socially appropriate manner. Sometimes it may be apt to be assertive-passive and sacrifice your rights for the demands made by someone you love. Sometimes it may be apt for you to be assertive-aggressive with a pesky family member. And so on.