When do you decide to sell?

I understand that most people recommend not timing the market and simply holding – but there comes a day when even these people sell to reap the capital gains.

So my question goes out to both these people AND those who trade more than passively, when do you PERSONALLY decide to sell? At which point do you decide “ok this is what I’m comfortable with earning” – is it strategic (as strategic as one can get), is it arbitrary, or is it based on life decisions (e.g. “I need a new car, let me get this”).