ok, this is how i see it. Don't you remember that Goku wasn't like other saiya-jins? Goku is a pure hearted saiya-jins, he still may enjoy fighting like all saiya-jins do, but he still has a good heart. thats how he turned out to be like he did, (to remind you again, this is just my theory, i'm not really saying thats why.) then vegeata was the same way, except without the pure heart, all of a sudden, he didn't care anymore, he didn't care about living, he didn't care about surpassing goku, he just didn't care anymore, then snap, there he went SSJ. Future trunks went ssj almost the same way goku did, gohan died and he snapped. gohan went ssj because he was disappointing everyone, and also with a little help of his hidden power. again, this is just how i see things.