What is the motivation model behind StackExchange? I noticed that I didn't sleep yesterday for no reason and spent the whole night to answer questions in one of the communities in here. Why did I do this? I still come here  every hour and check if someone chooses my answer or asks me something. There is a vague feeling. I'm like people who are on Instagram or Snapchat all day. I'm trying to understand what the reasons are behind this.

As far as I know, there are two general types of user here. People who seek help and people who want to help others. The motivation for the first group is obviously urgency of finding an answer for their question. However, the second group doesn't need to bother. They got no problems. What are the psychological factors that make those people help others and contribute to the community?

I know the reputation system has a big impact, but I think giving points and badges are not enough. I believe there are some powerful gamification elements such as unlocking privileges. What actually keeps people here? Is the model of StackExchange, which is depended on reputation points, sustainable? If all reputations were reset today, would people still come here to answer questions? Or there is something more, I don't see?
The short answer is that anyone who gives you a short answer should be treated with suspicion.  ;-p

The StackExchange was founded in 2008, and has seen a lot of changes since that time.  That has provided the science community with little time to study the phenomenon in much detail.  As such, there are many many theories, but little consensus, and no comprehensive model in place.

There are several related articles on Wikipedia - one thing I can recommend for now, is to keep an eye on them.  Wikipedia tends to be updated with new evidence-based findings over time, so as knowledge progresses, it will likely be featured there:


Motivation for creating user-generated content
Motivation for online participation
Motivation for contributing to Wikipedia
And in particular: Motivation for contributing to Q&amp;A sites


Speaking of Wikipedia, it is clear that points and badges are not necessary to motivate contributions, as many crowdsourcing systems succeed without them.  Presumably, social psychology (eg, recognition, identity) plays a big role, as does intrinsic motivation (eg, ideology, curiosity).

However, what would happen if such extrinsic rewards were removed is a question about a controversial phenomenon called the overjustification effect:


  ... gamification refers to the application of game design elements to
  non-game contexts ... by providing symbolic rewards such as points,
  badges, or virtual currency. However, a number of academics and other
  critics have expressed concern that these rewards may backfire via the
  overjustification effect. ... these critics of gamification express
  concerns that gamified contexts ... might provide expected rewards for
  activities ... and therefore reduce intrinsic interest in those
  activities.


That is, if this effect is real and applicable to the StackExchange, then there is a danger that initially intrinsically motivated contributors might become dependent on the extrinsic rewards over time.  Complicated.

In reality, like any product or service on the market, StackExchange is subject to market forces, corporate decisions may be influenced by motivational models as well as user feedback, but are often made for other reasons, and therefore its sustainability is not easy to predict.  It is likely the case that the underlying motivational model is multi-faceted, with each aspect of the system contributing to (or detracting from) its success, interacting in a multitude of different ways with individual user attributes, and changing over time.