Because life will always find a way to kick you in the balls. Being smart/talented is cool sometimes, but it comes with its own set of drawbacks, especially when you're a kid. Your parents pressure you, your teachers pressure you, and you start to feel like you're never good enough because you can't keep up with the ambitions they've instilled in you. Separating your desires from those of others is problematic when you're in middle school. It also has a negative impact on school socialization, but that wasn't the only problem. And voila, welcome to depression at 15.
And just when, after several years of effort, you get your shit together, boom, your chronic illness flares up and now you're going to go blind in the next few years, for no reason, and there's not a single fucking thing you can do about it, simply because that's how this reality works — some things happen just because fuck you. I'd be willing to be talentless and as disgusting as pile of shit for a pair of fancy working eyes, but no one is offering me a trade.
So that's how you can be smart (and pretty + humble) and want to die at the same time.
Also, may I ask why it's so important for you to be smart? Most people are somewhere between slightly below average and slightly above average and feel fine. Even at the universities I attended, there weren't that many really smart people. I think average intelligence is enough to study anything, unless you want to do groundbreaking research on your own.