1. ...it makes me feel generous.
2. ...because I feel smart alerting others to what I discovered.
3. ...because I care about the outcome and want you (the creator of the idea) to succeed.
4. ...because I have no choice. Every time I use your product, I spread the idea.
5. ...because there's a financial benefit directly to me (Amazon affiliates, mlm).
6. ...because it's funny and laughing alone is no fun.
7. ...because I'm lonely and sharing an idea solves that problem, at least for a while.
8. ...because I'm angry and I want to enlist others in my outrage (or in shutting you down).
9. ...because both my friend and I will benefit if I share the idea (Groupon).
10. ...because you asked me to, and it's hard to say no to you.
Read the other 10 on Seth's Blog: I spread your idea because...