Not an answer to your question, but there are other examples of this sort of thing. Lycaenid butterflies secrete nectar as caterpillars in exchange for protection by ants [1]. Indeed, ants have symbiotic relationships with a bunch of organisms, including fungus [2], aphids [3], and over 100 species of plants [4].

[1] https://australianmuseum.net.au/lycaenid-butterflies-and-ant...

[2] https://en.m.wikipedia.org/wiki/Ant%E2%80%93fungus_mutualism

[3] https://en.m.wikipedia.org/wiki/Aphid#Ant_mutualism

[4] https://en.m.wikipedia.org/wiki/Myrmecophyte

There is some element of random chance.

A random mutation happened, and that mutation ends up dominating the gene pool because it's more successful.