Reasons for running  

Here's Neil the Ethical Werewolf paraphrasing Ezra's theory on why Gore won't get into the race:

As long as Gore stays out of the race, all the other Democratic candidates know that his blessing would provide a dramatic boost to their campaigns, while his curse could break them. So they'll make sure to do right by him, proposing ambitious plans to fight global warming. But if he gets into the race, there's no gain for the other candidates in being really progressive on environmental issues, because all the environmentalist support will go to Gore anyway. The strategic calculus then favors finding some way to undercut Gore's favored proposals. In the end, the best way for Gore to shape the race is to stay out of it.

This makes sense if Gore sees himself as a one-issue candidate, but while the environment is obviously his passion, I think he's probably considering more than just environmental issues if he's thinking about running for president. And didn't he just write a book demonstrating that larger scope? I don't know if Gore will enter the race or not, but if he's even considering it it's because he has bigger fish to fry.