Will Traynor is like James Bond: all guys wants to be him and all ladies want to be with him. Well, some guys want to be with him, too, but you get our point. But it's this envy-inducing nature that makes it so hard for Will to deal with the accident that leaves him paralyzed.
Before his accident, Will lived a full life in every sense of the phrase. He was a successful international businessman. He made love to foxy ladies on the reg. He partook in every extreme sport under the sun. Seriously, dude lived life to the max.
Ironically, Will's not injured by any of these extreme exploits; he survived all of those just fine. He's injured one rainy day when he's simply crossing the street—an irony he refers to as "God's little joke" (6.128-129).
That explains why Will has such a hard time adapting to quadriplegia. He was never the type of dude who was content to just sit around and think—he always had to be doing. But now he can't do very much at all, and so he experiences a whole lot of tension between his former dreams and his current reality. Here's how he frames it: "I don't do anything, Miss Clark. I can't do anything anymore but sit. I just about exist" (3.122).
That would be a tough thing for anyone to handle, but it's especially difficult for Will.
Will's current status doesn't make him regret his past exploits—it makes him treasure them even more. As he says to Lou, "I will never, ever regret the things I've done. Because most days, if you're stuck in one of these, all you have are the places in your memory that you can go to" (15.64).
While Will's memories undoubtedly cause pain, they're also the only thing that provide relief.
That is, until he meets Lou.
What draws Will to Lou? Is it her quirky sense of humor? Is it her overwhelming sincerity? Is it her willingness to talk trash back to him? There's certainly truth to all of the above. No matter which way you answer, however, Lou becomes the only thing that gives Will a sense of relief, no matter how brief. She becomes "the only thing that makes [him] want to get up in the morning" (18.204).
Even so, this love isn't enough to keep Will from going to Dignitas to undergo assisted suicide. In the same way that Will's memories simultaneously give him pain and pleasure, being with Lou simultaneously makes him happy—because he loves her—and sad—because he knows that he can never express that love as deeply as he wants. As he puts it, "I can't be the man I want to be with you [...] This kiss becomes...another reminder of what I am not" (23.133). He doesn't want an unequal relationship; he doesn't want to put Lou through a lifetime of having to be his caretaker.
It's a brutal catch-22 that he just can't overcome.
So, how should we react to Will's final decision? Do we criticize him for being shortsighted and closed off to life's possibilities? (Is he even being shortsighted?) Or do we support him because he's relieving his pain in the only way possible? There's no right answer.
Ultimately, however, the most important thing about Will's decision may be that it's Will's decision. It's the first thing he's chosen for himself in a long time. No matter how we feel about it personally, it's hard to say that anybody else should choose what happens to him but him.