I have only been diagnosed for a few months, but I was told the same thing.

I lead a very high stress life. At the time of my diagnosis, I was in the middle of a bad case of mania, which I believe was likely spurred by relationship issues I was having at the time.

Prior to this, I was being treated for depression, but I do believe it is possible that I would never have been diagnosed bipolar unless it were for this stress snap.