Tweet and Retweet, but why?

Sharing is such a lovely thing, but why do we share?
PSFK posted sharables on sharing today:

From Social Times: 7 Factors That Drive People To Share Content With Their Social Networks
“When you see content online, be it a video, an image, a game or a blog post, what is it that entices you to share that content with your friends online?  Here are seven main factors that drive people to share content with their friends on Twitter, Facebook and other social networks.”

Also, Seth Godin’s blog says I spread your idea because…
1.I spread your idea because 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 (Hotmail, iPad, a tattoo).
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.
11….because I can use the idea to introduce people to one another, and making a match is both fun in the short run and community-building.
12….because your service works better if all my friends use it (email, Facebook).
13….because if everyone knew this idea, I’d be happier.
14….because your idea says something that I have trouble saying directly (AA, a blog post, a book).
15….because I care about someone and this idea will make them happier or healthier.
16….because it’s fun to make another teen snicker about prurient stuff we’re not supposed to see.
17….because the tribe needs to know about this if we’re going to avoid an external threat.
18….because the tribe needs to know about this if we’re going to maintain internal order.
19….because it’s my job.
20.I spread your idea because I’m in awe of your art and the only way I can repay you is to share that art with others.