Mutual love and happiness is a much more complex experience than misfortune and unrequited love.
Happiness can be accompanied by anxiety, fear of loss and other complex feelings, while grief is an obvious road in one direction. Survive, drink, rethink and get out.
And happiness is a more difficult life task. Or even, say, a philosophical category. To be here and now, without the ability to cling to expectations.
Therefore, those who are not able to survive happiness are temporarily given unhappiness.
P.S. And remember, just changing your mind - we change the world together!