Why do people get addicted to/obsessed with stuff even when they know it's bad for them?

it's addiction i guess...there's something in our brains that is messed up - I'm sure someone smarter than me can explain in more detail.

nicotine

The rational part of the brain isn't in command in terms of priorities. Our wants and desires management parts can and will override the rational part. The gut flora may even have more influence over decisions than what one consider oneself. Basically the rational part is slow and deliberate while the other parts are designed for quick decision.

Check neuroscience videos around marketing and propaganda. Although not directly related, they explain how our brain mechanics work and how it affects us psychologically (of course brain science is still pretty green, but it advanced a lot in the last decades) and our decision making.

I imagine one would need lot of brain "rewiring" to truly act rationally over desires and obsessions and not just have the illusion of rationality over these matters. Maybe rising awareness of self through meditation could help. Basically slow down our wants and desires and let the rational part deliberate and recognize biases. There are treatment for addictions and behaviors, but they aren't preventive. I don't believe there is much real research or focus on this. At least most will fall into some sketchy self-help category to guru bullshit.

The thrill and excitement.

The excitement, as well as satisfaction and even comfort.