Members of different species often have much to offer each other because they can bring different skills to the partnership. In the relationship between the aphids and the ants, the aphids collect the juice from plants, and in this case honeydew. Once digested the aphids pass the juice to the ants through their backside, the ants love the sweet juice. The aphids do not have the ability to protect themselves from predators, so the ants in return for the sweet juices provides protection for the aphids. This kind of favorable asymmetry can lead to evolutionary stable strategies of mutual cooperation.
Symbiotic relationships are common among animals and plants.
There is one idea that is not scientifically proven, but rather a speculation that each one of our genes is a symbiotic unit. We are gigantic colonies of symbiotic genes. The other side of this is that viruses may be genes that have broken loose from colonies such as ourselves. When thinking about mutual benefits, problems may arise if there is a delay in repayment after giving a favor. This is because the first recipient of a favor may be tempted to cheat and refuse to pay it back when his turn comes.