Now that Gore endorsed Dean, many have tried to figure out the big reason- Why? Gore's accomplished alot in his life- I won't bore you with the details, but I can imagine only two more things he might want to accomplish. 1-Be a Good Democrat, work for his progressive ideals, maybe strengthen his reputation a bit after '00. 2- Be President.
I'm going to make an assumption now that many disagree with, but at the very least it makes the math easier, and I happen to believe it, so, while I grant it isn't fool-proof, I think it is at the very least very arguable. I'm going to assume that Gore thought that if he stood out of it, Dean was virtually guaranteed to get the nomination.
So, 3 possibilities about what happens in a Dean-Bush battle (for our purposes). 1- Bush smashes Dean. 2- Dean loses, but only barely (45+% of vote, 200 EV, etc). 3- Dean wins.
If Bush smashes Dean, then Gore couldn't have done much anyway for his liberal ideals. He slightly reduces his chances of ever becoming president, because he's attached himself to Dean, but it's arguable that had he done nothing, then Hillary would have been the frontrunner anyway to get the '08 nomination (andrewsullivan.com has a very good article on that).
If Bush wins a close victory, then, again, Gore's liberal goals failed. His possible presidential goals, however, have been strengthened. Now, going to '08, we have Hillary on one side, Dean, if he wants another try on the other, and Gore, who is moderate but has some pull with the Deaniacs. And, if Dean returns the favor, perhaps Dean won't even run, giving Gore an outstanding chance to win, with troops and a more moderate exterior.
If Dean wins, Gore's liberal goals have succeeded. Perhaps he envisions himself an Adlai Stevenson-type role, maybe with another chance at being a hero like Stevenson did in '62 with the Cuban Missle Crisis. His presidential hopes might have gotten weaker, as he'll be awfully old come '12. ON the other hand, if he gets State, he might still be able to do it.
To say this in another way, one could argue that Gore's goals is either to be like Stevenson in '60 or Nixon in '64 (not in the bad way- I just mean become president eventually). We don't know yet what will happen, but I think the party became stronger today for it.