The 30-year-old defender this week completed a free transfer to Vicarage Road after five years at Pittodrie.
Former Scotland international Severin has spent his entire career in his home country and is relishing the opportunity to test himself south of the border.
Asked if he was becoming "stale" by staying put, he told RedTV: "Of course.
"I said to myself if I get the chance to go at the end of my contract, I'd go.
"I've had five good years at Aberdeen. I had five at Hearts as well - so 10 years playing against the same teams.
"In that time, I could have played teams like Motherwell maybe 40 times.
"It's good to get a change and hopefully I can become a better player playing against better players."
Severin, who was Aberdeen captain for the last two seasons, revealed his move to Watford was a done deal some time ago but the announcement had to wait until new Hornets boss Malky Mackay returned from a coaching course.
"I actually knew quite a while ago when Malky first got the job," Severin said.
"But him being on his coaching badges, the club just had to wait until he was back to announce it."
Severin also had other options, adding: "There were a few teams interested but none of them would ever commit until about July 1.
"Then Watford came in and I was delighted to sign for them."
He is now looking forward to trips to the likes of St James' Park, the Riverside Stadium and the Hawthorns next season, saying: "You've got to go to Newcastle and play in front of 50,000, Middlesbrough, West Brom - the teams that have come down - and Leicester, who have come back into the division as well and who are also a big club."
Despite wanting to leave, Severin insists it was a hard decision to walk away from Pittodrie.
He said: "It was difficult. I've got a young family as well; I've got to think of them.
"I've had five good years, a great bunch of boys in the training room, and I hope they do well this season."
And Severin had some advice for Mark McGhee, who replaced Jimmy Calderwood as Dons boss this month.
He said: "The one thing about the old manager is that he didn't win a cup or get to a final and I think if he (McGhee) can do that in his first season, he'll be judged as a success."