Fine.

If fruits are produced by plants which animals then eat and spread seeds, why are lemons so sour?

If fruiting plants have, over time, developed traits which make their fruit attractive to animals to eat, and thus spread seeds around then, why are lemons so sour?

It's a goddamn ELI5. It comes from a place of wanting to understand something but having no background in it. OP wants to know why a species would evolve something distasteful in a part of its biology which has evolved to fit niches where being tasty would be more advantageous. But OP is unaware of the error of thinking his post title could convey if overthought by one of the internet's many misanthropes. OP very probably understands that plants and animals don't just decide what their traits will be, because OP is very probably not a literal five year old.

It's true that it's wrong to think of individual plants and animals as free actors, but when we're talking about a larger population or species we can make some broad generalisations. They're not correct on the individual level, but they're very convenient when we explain things in simple, plain-english ways.

For instance, when we measure the success of a species by how good it is at reproducing we assume the goal of the species is to reproduce. It would be wrong to say that animals eat fruit in order to poop out seeds, but saying that a plant species produces fruit so that animals eat and then poop out the seeds is close enough that it's probably good enough for ELI5.

The species doesn't produce fruit because of intent, it produces fruit because it has evolved to do so. Evolved traits aren't intentional, but they frequently interact with other organisms and environmental factors to the benefit of the organism.