It's a problem I have. I have a difficult time telling people "no" when they asked me to participate in a project, or for my help. Part of the reason is I don't want to seem uninterested in their project or selfish with my time. Also, chances are if they're asking to participate, it's something I'm good at, familiar with, and or enjoy.
I know I am far from the only one with this problem. It is at least partially why so many of us struggle to find the time to get all things we want to do, done. We are simply over loaded.
We need to remember that it's okay to say no, and we all need to say it more often.
It's can be hard to not offer others your help. But when your help takes away from the tasks and projects you are responsible for completing, you suffer.