It’s easy to be unhappy: to complain, be negative, feel self-pity. So easy in fact, it can creep in without warning. It can seep in, unnoticed like a leaky pipe can damage a house drip by drip.
I don’t know if there’s any way to prevent it, but seeking to mitigate unhappiness is a good place to start. You begin by working on discovering the drip before the damage is too vast.
The good news is that while happiness is much harder to attain, we need to remember that the only things that are ever worth doing are difficult to start and to maintain.