It's hard to pick any single one, but in the end I went for laziness. His life could be so much better if only he'd haul himself up off his ample backside and do something to improve his lot. Take, for instance, his sweetheart search. If he lost weight, showered, smartened up, got a job and actually tried approaching women, he might have some success. But what the sagas have shown is that the moment he has to make any effort, he'll do the bare minimum, and if that's not enough, he'll shrug and retreat to his hugbox. It's the same with everything in his life. The moment something becomes an effort, he'll invent some lame excuse, probably involving stress or autism or lack of money (that went on Lego). People have tried to help him, but it's always the same story. If they don't do everything for him, he's just not interested. I'd also say that egotism, stubbornness and entitlement play into this, but they're subservient to his life goal of doing fuck-all.
I honestly think that if he'd applied himself, he could have been happy. He'll never be the high-flying billionnaire games magnate with a supermodel wife waiting on him hand and foot. But he could have got himself a job, plodded along, saved some money, moved out, got himself a girlfriend maybe. As it is, he has literally no reason to even get out of bed in the mornings, and his life has been such a mess for so long that his chances of gainful employment and a sweetheart recede further and further into the distance.