The reasons are complicated and evolutionarily symbiotic. Lure a "pollinator" to protect and spread it. Avocados were the diet of some unknown animal that no l

The reasons are complicated and evolutionarily symbiotic. Lure a "pollinator" to protect and spread it. Avocados were the diet of some unknown animal that no longer exists. Avocados shouldn't exist except for humans preserving and exploiting them. Well, look at capsicum. Birds can't taste it but mammals can. Even so, most deer and some humans sure do like jalapeños. Some compounds are deadly poisons to particular predators while merely annoyances to others. Beyond deadly poisons, there are deterrents like emetics, anticoagulants, dissociatives, and paralytics aimed at defending from particular predators. These are evolutionarily-conserved because they add to the fitness of a species. Nature is a hacker: it reuses (mutations) a lot of pathways and molecules for different purposes. We just hack compounds out of nature intended for other functions that also happen to serve our purposes at therapeutic doses.