There are different situations where I would like to ask a friend or a roommate to do something or to stop doing something. 

For example, I have a roommate that sometimes doesn't wash his dishes and I don't like it. How can I politely ask him to wash the dishes without being preachy and without offending him? 

Another issue I have with this is that anytime I ask him for something, I feel bad or even guilty, even though I assume what I am asking is pretty reasonable. 

Is it normal to feel bad about asking other people for things or am I doing something wrong?

P.S. In case the cultural context is important: we live in the apartments in the United States, but originally we're from different countries, same age.