There are probably at least 350 reasons why Damon and Elena should be together. These are just a few.
You and I, we have this thing. An understanding.
At the core of any amazing relationship is rock solid trust. They have that; no matter what Damon's done—four episodes ago it was killing her brother—Elena knows she can trust him with her life. Likewise, he knows that she can be trusted to defend him when he is not in the wrong—see "Bloodlines" or "Brave New World." That gives him extra motivation to be the man she sees in him.
The trust between them was earned. It's the fact that he didn't compel her in Atlanta. It's the fact that she offered sympathy for his loss of Katherine both in "Friday Night Bites" and in "Fool Me Once." It's the fact that from the moment they met, they've been able to see through each other's walls to what is lying underneath.
Damon and Elena embody the saying that "You come to find love not by finding the perfect person but by seeing an imperfect person perfectly."
They are evenly matched. Sure, she's a 17-year-old human and he's a 160-something-year-old vampire. But she's not scared of him. She'll smack him when he deserves it and call him out for his b.s. every single time.
Likewise, he lets her make her own decisions and he has her back always—see "Blood Brothers," "Isobel" and again "Brave New World"—but he never lets her forget that she is the one who has to own her actions; good or bad.
They bring out the best and the worst in each other but it's never boring. It's messy, and it's complicated and it's intense and it's scary as hell for both of them. He has the power to make her feel more than she's ever felt. She has the power to make or break him. But the number one reason they should be together is because what they have is real. And when it's real, you can't walk away.