Sun Elves are often very proud and may consider themselves to be the true defenders of Elven faith. In extreme cases, this may even make them look down on other elves let alone the non-elves. Maybe your Sun elf was born of a family with rather extreme views in that regard, yet does not share them himself and wants to break away from it?
This is all related to the rather touchy subject of Elven purity which you will find in many elven cultures. Some (usually Moon Elves) are very tolerant towards other races.
On the other end however, there exists a group of elves known as the
Eldreth Veluuthra(clicky) who strive to inflict as much harm to humans as possible, for example. Not all elves support them, let alone openly, yet there are elven families who do secretly fund them through coin and other valuables that they might need to support their cause.
I used the "Elven purity" thing before for one of my characters and I liked it very much. Maybe that's something you'd wish to incorporate into your proud Sun Elf somehow or his family?