John is moving to PRO12 rivals Scarlets after a nine-year spell with Glasgow.
John told the Evening Times: "It was a really tough decision to make and there is no doubt it was the hardest of my professional career, but I think it was the right time for me to move on.
"Nine years is a long time to be at one club. I have seen so much and experienced so much with Glasgow but there comes a point when you need a fresh challenge.
"I have always had a very high regard for the Scarlets and how they play their rugby and, just like Glasgow, they made the PRO12 play-offs and they will be looking to kick on again next season. I am looking forward to being part of that.
"But you never know how things will work out in the future. I am only 26 and you can never say never, maybe I will come back to Glasgow one day."
He went on: "I am leaving with so many great memories and it was just such a pity that we did not make the play-off final.
"Really that is my one big regret, that we did not win anything at Glasgow this season. Because we were more than good enough to.
"But I think we can be proud of our performance at Leinster on Saturday and I am sure the boys will kick on again next season.
"Shade Munro has been a constant in my time at Glasgow and I owe him a lot. To have worked under first Sean Lineen and then Gregor (Townsend) has been fantastic while our defence coaches Gary Mercer and recently Matt Taylor have also been great with me.
"I will miss Glasgow and everyone at the club but I will relish the games with them next season when I am in a Scarlets jersey."