Why do we like to be "The one who..."?

Recently a friend and I were talking about something that spurred this question.

Her boyfriend and her recently got a roommate, His best friend. She and he have a lot in common. They both have seen numerous movies, old and new, that her boyfriend hasn’t seen. The friend and her both love the same foods, snacks, even anime (which he cant stand). They can get into discussions that he cant join into because he knows nothing about or isn’t interested in what they are talking about. Its like they have their own little world.

Now before you get ahead of me. The boyfriend is not jealous in ANY WAY. He knows that she loves him and only him, she doesn’t find the friend attractive in anyway and that his friend would NEVER cross that line.

Here’s where the question comes in. The boyfriend said to her one night ”you and ___ would make a better couple than we do with all the things you have in common and we don’t.” Her response was ”No we wouldn’t, you and I do because though him and I have a lot in common, I like that you and I don’t. I like being the one to introduce you to new things. If there’s something you like that you didn’t before me or didn’t give a chance to but did because I like it and now you do too, than I was the reason. I like being the one that turns you on to new things, the reason you know about ’ this and that’, the one that taught you or got you to like something new. With him, he already knows and that would get old.”

So back to the question. Why do we like to be that one who… She told him she likes to be that person but not why.

Is it because after we “break up” when ever that person sees or thinks of something that we introduced them to they will always think of us? Or is it less selfish than that? Is it just because it makes us feel proud? Or happy that the one we love enjoys one more thing in life and that makes us happy, so we want to have a bunch of chances to do that?...Why?

Please REAL answers ONLY, NO rude, disrespectful, jokes or a#%hole comments.