Critics who point out that Sachin Tendulkar is 'selfish' in his desire to want to continue are spot on, except that this quality of selfishness is what we ought to emulate and instill in our self. Or rather not. That's how we are naturally.
Deriving from Darwin's theory of evolution - whose ardent proponents hold intra-special instead of inter-special competition as more relevant - intra-racial competition is more important to us than inter-racial competition. That is, we compete and fight more with people who are closest to us than with people of other races. The road to growth and victory is actually associated with outwitting people who are more like us. This is our true nature. Even among animals, a lion will not accept another lion in his territory. He will be keen to kill him, rather than share and prosper together. Or sacrifice his position as the master.
What many of Sachin's critics are tending towards may be termed as "group favouritism", which puts forth the theory that animals function according to the interests of the group; they put group ahead of the self; the individual's self-interest is kept secondary. This theory was put down several decades earlier but finds a place in modern parlance. It manifests itself as sacrifice - and holds that man is essentially a team player. It further argues that even though 'some' humans keep self-interest over group interest, the quality of 'humaneness' makes them sacrifice.
However, it is to be noted that sacrifice, in actuality, is a quality that is best performed for the world, by which I mean that sacrifice is also a selfish quality. We sacrifice for self-fame. We give up to get more.
Thus in the immortal tale of Heer-Ranjha, Ranjha sacrifices comforts to get his beloved Heer. And thus the televised drama of a certain politician sacrificing the prime-minister's chair in 'national interest'.
Sacrifice and group favouritism are not base human qualities, no matter what we have been fed by our school textbooks. As such, those aspiring for sacrifice are actually selfish and manipulative. Exceptions occur always among people who quietly sacrifice, but these do not form the rule. And as individuals we rarely seek out such hidden gems.
Therefore, It is perfectly fine for Sachin to want to continue playing cricket. Even as a pure desire, free from the question of form and fitness, it is perfectly 'human' for him to want to continue. That he keeps playing well is a quality that we must admire and seek in ourselves.