When you do seva don't think you are doing a favor for somebody. You have done the seva and it has rewarded you immediately.
Its reward is for sure and is always more than your doing. Your expectation of reward for the seva turns the seva into labor.
If you think you have done a lot, you will do very little; if you see you have done a little, then you will do more.
Seva is even when you don't see an immediate reward there is no complaint; labor is even after an immediate reward, there will be complaints.
Be grateful for any opportunity to do the seva.