I agree that he was happy and carefree with Pumba and Timon but when
he went back to pride rock he had more responsibilites and he had to
fight Scar for his home. Also it must of been somewhat more stressfull
to have a daughter and wife than being with Timon and Pumba all those
years.

I'd say Simba was more carefree when he was with Timon and
Pumbaa, but keep in mind that was a shield to hide from the
sadness and guilt he felt. When he became King He was free
of that guilt because his father's murderer had been
punished. His home land was saved, the love of his life
became his Queen, and then he became a father.
So even
though Simba has more responsibility he was also much
happier at Pride Rock than in the jungle.

i sorta agree he had more responsibilities than when he did
and he had to raise kiara too