RE:Becoming unbearable...It depends on what kind of investor you are.
I'm not into flip-flopping, so I usually hold stocks when I think there's upwards (long term) value. It's no secret that I believe Altagas is worth 28 at the moment, a figure that could increase further as Altagas keeps making good moves, so obviously I'm not selling a share.
That's me though, any investor thinking the stock is close to its worth and sees opportunities elsewhere should likely sell.
There are also traders that will try to sell and buy lower, which they may succeed at sometimes. I used to buy/sell more often years ago, but after comparing the return of two of my portfolios, one in which I was more active and the other more passive, I have found that I get better long term results sticking with the long term view (over buying/selling frequently).