The easy answer is that Sherlock tells us why he’s a detective: it’s a replacement addiction. Either he solves crimes, or he gets high. Apparently being a scientist or a philosopher wouldn’t provide the same kind of high. There it is! Done and dusted, right? Should we believe him?
Much like his creators, Sherlock lies. He lies to project an image of himself that he tries to embody, even through it really doesn’t fit. So perhaps we shouldn’t take his explanations of his own choices at face value.
He obviously likes to solve puzzles, and he has a gift for it. But so do philosophers and research scientists. If all he wanted was to solve complicated puzzles, academia would have been the right place for him.
As John noted in TSOT, Sherlock is a drama queen; he likes it when the stakes are high and the audience is holding its breath. Being a consulting detective, where life and death are at stake, fits the bill in the stakes front.
Sherlock has an odd relationship to his audience. I genuinely can’t tell if he loves it or is completely indifferent to it. You can argue both ways, which is a bit confusing, but I think points to Sherlock’s inner conflict about his need for love and attention.
Very often Sherlock doesn’t seem to care what other people think of him. He insults clients and doesn’t appreciate their gifts or their gratitude…except when he does. Extra chips and meals on the house: he doesn’t turn those down. In fact, he appears to seek them out. Sherlock didn’t have to go to Angelo’s in ASIP. He didn’t have to let a flat from Mrs Hudson, either. He doesn’t even really need a flatmate when it comes down to it, as far as we can tell, except that he seems to want one. As much as Sherlock doesn’t want people around him and doesn’t care what people think of him, some part of him appears to.
Is it a little war waging inside him, between wanting love and rejecting his desire for it, being so certain that he is utterly unlikeable? If he really didn’t want the broad attention and the gratitude, he could have been scientist or a philosopher, and communicated with the outside world exclusively through obscure publications. But he has not opted to do that. He’s opted for a desperate audience with its eyes trained on him.
Sherlock doesn’t want to need people, but he does. He doesn’t want to want love, but he does. He doesn’t want to be driven by people’s opinions, when so often they are negative or dismissive (Sebastian Wilkes, Donovan, Anderson). He’s a high-functioning sociopath, he says, and doesn’t need affirmation from normal people. Except that he’s not, and he does.
For all that, the high-stakes puzzles and the eyes of an audience of thousands isn’t enough for Sherlock. He is still on the edge of returning to his drug addiction at the start of series 1, and there are strong hints that he’s actively suicidal as well as reckless with his own safety. If solving crimes is meant to keep him on the straight and narrow, I’m not sure it really fits the bill. Close, maybe. But not quite enough. What does help him is the admiration of someone whose admiration he actually wants. Being loved of someone he loves seems to do wonders for Sherlock. That’s the real thing, that’s what’s he really needs.
I know it’s a romantic answer, but I think what we can deduce about Sherlock’s heart is that he has one. Sherlock is a lonely man who longs to be incapable of loneliness. He desperately wants to love and be loved, but he has repressed his desires as much as he can, trying to turn himself into someone who doesn’t want any of that. He doesn’t want to be himself; he wants to be a different version of himself, one who needs nothing and no one.
But he can’t erase the core of his character. He is who he is, and even his incredible brain can’t turn him into someone he’s not. So Sherlock’s loneliness and desire to be loved, so long unexpressed, squeeze out of him in a million other ways. The drama, the high stakes, the puzzles, the admiration, the audience, and the shower of gratitude all help fill that deep well of need inside of him that he won’t acknowledge. That’s why I think Sherlock is a consulting detective instead of a scientist or a philosopher. Because he wants to be loved.
Yeah, that’s definitely the romantic answer.