am181d
Adventurer
I think whoever answered "hybrid vigor" as a reason for the +2 Con in the other thread wins.
That sounds sciency to me, and therefore lame.
The obvious answer is that the +2 Con represents extra pluckiness. (Or, if you want to play a badass half-elf, extra cojones.)
Remember that hit points in 4th Edition are as much about morale as actual physical toughness. Their Con and Cha bonuses are both a testament to their strong personalities.