A few years ago I started noticing that a lot of people don't want to share their knowledge. Usually, the reason is very simple and sounds like, "I don't want to share my knowledge because I spend a lot of time getting it".
You might never hear those words really said, but you will certainly feel them very clearly. If you work somewhere, you might experience something like this quite often while talking to your co-workers.
I think the main reason why we should share information is: the knowledge you know, actually, isn't yours. You know it only thanks to other people that decided to share information (through books, articles, their own experience, etc.) with you in the past.
In fact, we would have and know nothing, unless somebody created it.
There are more practical reasons why we should share knowledge, but I like the above idea the most. It brings the understanding that I'm a part of something bigger .