When people criticize my novels, a common complaint is that I spend too much time explaining my characters with back story, memory, etc. I do indeed do a lot of that, though I never think of it as explanation. I think of it as something interesting that happened in the character's past, which I want to get to write about. Sometimes I can't bear to edit it out later, even if it comes to seem gratuitous.
I'll take this a step further, though, and suggest that maybe protagonists don't need to be developed all that much, period. Maybe all that matters is a voice for the reader to buttress with their own assumptions and imagined detail. Secondary characters, you need to develop better, because they aren't who the reader is identifying with. But the protagonist, quite often, is there to be filled up with parts of the reader's self. All the writer needs to do is hint, and the reader will supply natural psychological explanations based upon their own experiences and feelings.
Of course in some cases the back story is the point; in some cases the novel consists of the protagonist explaining himself, or trying to explain himself and failing (e.g., Kazuo Ishiguro). But for a lot of books, it just isn't necessary. Think of Chandler's Marlowe, or Denis Johnson's Fuckhead, or just about anybody in Flannery O'Connor.
Ultimately it's a matter of confidence in your writerly ability to evoke a personality. Like the Rembrandt drawing above ("The Single Greatest Drawing Ever Made," according to David Hockney) it's as much what the artist chooses to leave out as what he puts in.