Because in most games if you make the first move I'm one step closer to winning.
Does that sound bad?
It really isn't.
It just means we're a move into the game where I haven't already fucked up. When you make the first move I have an extra moment to think and the chances I'll fuck up my move are cut down drastically.
Now honestly between the two of us, you've gotta be the smarter one because you made the first move. You said lets start this game and see where it goes, and whatever you think will come of it you were willing to jump in feet first.
If I would have made the first move and it was the wrong one you'd have regretted wanting to play. There would be no fun had by anyone. You wouldn't be able to think through a valid counter move but you'd already be second guessing yourself, you wouldn't have already made a move though and your chances of success are already cut down. You're starting at a disadvantage and that can be challenging for defensive people.
So please, feel free to make the first move. I dare you. Because the more fun we have together, we've already won.