I'd guess it could be that the importance of some of the dreams people tend to have when they're kids, or at least just starting out in life, can start to diminish as people encounter things, other people, new dreams based on their new level of maturity move in and kind of make those old dreams seem less important (maybe even not all that important or appealing at all).
People grow, find those things in their life that are far more important to them than achieving their own "life's dream" , and either build new, updated (sometimes even more important, although different) dreams or else just abandon the idea of "a dream" altogether. What started out as dreams can turn into a case of "Well, that might be nice if I can do it," or "..would have been nice if I could have done it, but isn't something that's all that important to me now anyway, because I have so many people and things in my life that I care about so much more than I ever did that dream I once thought was so important."
More youthful life dreams aside, I'd guess that a sense of responsibility might stop a lot of dream-pursuing. I really tend to think, though, that people just change as they grow and their life changes. Circumstances, illness, etc. stops some people. I don't know if that would Number 1 or not. Not being willing to make some sacrifices that would be required could be a big one. That goes back to people's thinking that some dreams aren't worth the sacrifices of other more important things. Sometimes I think that the less well rounded and whole someone's life is, the more likely it is he'll a) have little to sacrifice if he pursues a dream, and b) see his dream as more important than someone else would see a dream.