Thor had a really bad year, in not one but two movies whose events unfolded back to back (at most a few days apart). In Ragnarok he
- Lost his father.
- Saw Mjölnir smashed before his eyes.
- Got kicked off the Rainbow Bridge.
- Got captured and made into a gladiator, while Loki sits in the ruling class.
- …all while knowing Asgard is being decimated under Hela, and he can't help.
- Got roundly beaten by Hulk in an entirely fair fight, uh huh.
- Caused Ragnarok.
- Saw Asgardians decimated again by Thanos, including Heimdall and his brother (?)
- And then all the stuff that Stark07 mentions in their answer that focuses on Infinity War.
…and worst of all, found out his Avengers password wasn't "strongest Avenger" but "Point Break"!
Five years gone, we find him depressed, drunk, obese and out of shape, trite, petty, and hiding from the world. Even on the mission he was more interested in alcohol than the job.
"Worthy?" Was a very good question.
Plus I think he really wanted to dual-wield Meow-meow (its proper name) and Stormbringer. Or at least loan it to Cap, to find out if Cap was faking in Avengers 2. It occurred to him that it could be borrowed and returned just as the Infinity Stones were.