Expectation of some gain is generally seen behind motive of service. But how does one achieve satisfaction through service? We find that there are diametrically opposite opinions about this. But the basic reply is that one offers service just for one's own satisfaction,
Atmananstu Kamaya Sarva priyam bhavati – Brihadarnyak Upanishad
Even though apparently the person to whom the service is offered is benefited by it, but service is offered for self-satisfaction and the first beneficiary is the person who offers service and not one who receives.
It may be out of (i) compassion for the people or (ii) a sense of pity or (iii) even a sense of kinship, fellow-feeling or fame or brotherhood or (iv) a desire for popularity, name and fame or (v) an ulterior motive of obliging the people for some future benefit such as election, etc. Thus it is ego-satisfaction or the feeling of superiority complex arising through patronage. Therefore, there is a give-and-take, a barter, a mathematical precision in receiving the returns approximately for the service rendered. There is a dormant desire to serve others for one reason or the other and this desire is fulfilled through the act of service. So desire is the birthplace of service.