Well , english is not my first linguage, but I am gonna to do my best. Reasons that I think why DE is endgame:
Elena and Matt broke up because it wasn't passionate , but Elena and Stefan love wasn't passionate too.
Stefan and Elena fell for each other " instantly" and they start dating in the seconde episode without knowing anything about each other, and then they start to see the worst of each other, Damon and Elena know the best and worst of each other because they were enemies , then friends, then best friend and then boyfriend and girlfriend , they have journey and Stefan and Elena don't.
Damon makes elena much funnier and much herself when she is with him, with Stefan she isn't completely herself and I think that she thinks that she always feel that she needs to be perfect.
For me Stefan is Elena puppy dog, because when he doesn't agree with elena he makes the thing she wants even he doens't agree, some people see respect I see fear, Damon doens't care to be the bad guy, for me he does the right thing because elena is not always right And in 3x22 Stefan saves Matt instead of elena, what she wanted but she not wanted to be a vampire, if was damon on the bridge elena would be alive and had the life that she always wanted. In the 3x18 elena agrees with damon logic she zei that what damon wants doens't mean that he is right and she wants to do the right thing, like Damon does.
Elena wants one love that consumes her, she wants a little danger , she wants to have fun she only gets that kind of love with Damon not with Stefan of Matt.
In 3x05 Klaus zei that Stefan and elena love will never die, but the true is that at that time doesn't know anything about love, right know he understands better thanks to Caroline and in 4x12 I think, Klaus asks to damon what he did to elena love him and zei that he only sees Stefan and elena in the end blah blah blah, he zei that because he is in love with Caroline and he think that she will always love tyler and he think that is the reason that elena and Stefan will be together, but the true is that elena and damon love is diferent of Caroline and tyler love, klaus wanna know why elena loves damon because he wants Caroline to love him.
Elena zei that her love for Damon is the most real thing she ever felt her entire life, she never zei that to Stefan.
Elena kind put damon in season 4 in the front of her friends, she didn't do this with Stefan.
Stefan makes elena feel like a broken toy , damon makes her feel meer alive than ever.
Elena was the only person who saw good in damon and she never stopped until he feels something.
Damon and elena bring the best of each other.
Damon loves elena either way, even at her worst, being a vampire of human, Stefan doesn't.
Damon in the end is who keep her alive.
And because of their songs, like "Never let me go", "Belong" , " Holding and letting go"...
this are some reasons why I think DE is endgame:) hope u guys liked :) and put Stefan and elena together again doesn't make any sense elena improved so much since the 1 season she isn't the same girl who died on the bridge, she is much stronger