The importance of selfless seva is highlighted by the Guru in this verse: "ਸੇਵਾ ਕਰਤ ਹੋਇ ਨਿਹਕਾਮੀ ॥ ਤਿਸ ਕਉ ਹੋਤ ਪਰਾਪਤਿ ਸਆਮੀ ॥ One who performs selfless service, without thought of reward, shall attain his Lord and Master." (SGGS p 286) The Guru states that one who perform selfless service without desire for reward will certainly attain liberation. The need to be "desire-less" ("nehkaami") is critical in making this action fruitful. When one does Seva, one should just do it without any thought for a return - think of it as a duty to the society.
"ਸੇਵਾ ਕਰਤ ਹੋਇ ਨਿਹਕਾਮੀ ॥ ਤਿਸ ਕਉ ਹੋਤ ਪਰਾਪਤਿ ਸਆਮੀ ॥