Its about wisom behind the dua. Quran guide us to make dua by focussing here and afterlife. when a person make dua for other person then he or she may not be focussing on hereafter. Its against the wisdom taught to us by quran.
However, there is nothing wrong in it. Duas are accepted based on Allah overall scheme, wisdom and justice.
I have observed that, we human actually dont have real understanding of what we want we just get emotional and ask for something which might not be beneficial for us.
See part of verse 2:201
“Our Lord, give us in this world [that which is] good and in the Hereafter [that which is] good and protect us from the punishment of the Fire.”