The transition from happy-go-lucky Koffing to depressed single dad Weezing is truly sad once you watch it happen instantly in GIF form - and it's pretty clear what happened.
Koffing was the cool kid in school - he smoked even though he wasn't supposed to, and everything was going great for him.
Then he had a kid - and a kid ATTACHED to him no less. The kid was plenty happy, but the sudden responsibility meant Weezing's former hard-partying days were done. That, and the years of smoking were finally taking a toll on his health (hence the name "Weezing"), led to the depressed, overweight sad-sack you see before you.
Weezing - the saddest Pokemon.