A person sometimes do selfless favour only to get remember, or craves to receive. If one doesn't acknowledge his/her contribution. Person feel low and inferior. So my question is is being altruistic is a sign of low self esteem?
Not at all. Altruism activates signal to reward and pleasure areas of our brain, makes people feel better. You can be a leader, embrace altruism, and be assertive.
Low self esteem leads to avoid assertiveness and to put other people interests ahead of yours, but this is not altruism, this is just low self esteem.