All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
If you are requested to supplicate for someone, then it is recommended for you to honor his request and supplicate for him even if you do not have the desire to do so, because there are two benefits in supplicating for him:
1) A benefit for you; if you supplicate for him an angels would say: "Aameen, and for you the same." It is confirmed that the Prophet said: "The supplication of a Muslim for another Muslim brother in his absence, is accepted by Allaah, as he has an appointed angel at his head, and every time he supplicates for his brother to a good effect, the angel would say: ''Aameen and for you the same.'' [Muslim] The supplication and the saying of Aameen by the angel is more likely to be accepted.
2) A benefit for the person for whom you supplicate; Allaah may answer your supplication and bless your Muslim brother with the good you supplicated with. If this is achieved, you would have helped your brother in attaining good.
Nonetheless, it is not correct to say that when someone asks you to supplicate for him, you are obliged to honor his request, and no scholar has ever said that a person is obliged to do so.
Allaah Knows best.