Isn't love a "happiness of stupidity" to some degree? It defies rationality, and the odds of it lasting aren't good statistically. Should you believe in it, then?

Love is a feeling, an experience. It is not an experience of something; it is simply an experience, like pain is, or the perception of colours. There is no propositional content to it. It is not something you can believe or disbelieve; it is not that sort of thing. There is nothing to believe in.

As with pain, there tend to be propositions associated with it, but not implied by it. Perhaps this toothache is a sign of tooth decay reaching a nerve, for which the remedy is to have dentist repair the tooth. Or perhaps it is an inflammation that a course of antibiotics will quell. There's no point in believing one or the other the moment you notice a toothache. Instead, you need to find out what has caused it, and then you can conclude on the appropriate action.

"Love is not love / Which alters when it alteration finds", writes the poet. This is an empirical test to assess one's love, not an injunction to cling to a fantasy about the beloved and shun disenchantment. True love is something to be discovered, not assumed. It is true if you observe that it continues while you get to know the beloved. It is a matter of noticing the feelings as they evolve, not of blinding oneself to the possibility that they will change.

"If this be error and upon me proved,
I never writ, nor no man ever loved."