Many animals work together because both parties can benefit from a "deal".
For example, many large birds forage food for themselves. But not the egret.
Cattle egrets have learned to land on a water buffalo's back and pick out insects. The egret gets something to eat while the buffalo gets the insects out of it's fur.
Another exaple is the tiny Egyptian plover bird. It often spend their time perching in a crodile's mouth! The plover picks out leftover food between the croc's teeth while the big fella just waits patiently, forgoing a potential midday snack.
That's because while the plover is getting it's fill, the crodile is getting a routine dental checkup while the plover is cleaning house.