Why is it forbidden to grieve for more than three days after the death of someone? Is it in the control of a person not to grieve?
It is certainly not forbidden to feel aggrieved for a dead person for more than three days. To grieve for a person is not in one's control. In fact, it is so natural that the Prophet (sws) himself wept the death of his son, Ibrahīm:
When Ibrahīm was about to die, the Prophet (sws), it is reported, picked him up, kissed him while tears were coming out of his eyes. One of his companions asked: O Prophet of Allah, do you cry also? The Prophet replied that this crying was a blessing and then while weeping remarked: Eyes shed tears and the heart is aggrieved but we shall only say that with which the Lord is pleased and we are sad at being separated from you O Ibrahīm! (Bukhārī, Kitābu’l Janā’iz)
The thing which is desirable is that a person should come back to his or her normal life routine by not later than three days. He should not just sit and sob after this period, but should try to adjust to the new situation and try to follow his general routine.