In "Ethics of the Fathers," Hillel states "Don't judge another until you reach his place;" meaning, until you have been in the exact same position. Therefore, you can almost never judge another's feelings.
Even if one feels sure that the other person is over-reacting, he should carefully consider if, how and when to express it. As Rabbi Shimon ben Elazar says, "Don't try to appease your friend at the moment of his anger, and don't try to comfort him when his dead lies before him." Trying to cheer someone up at the wrong time, or to tell him he's over-reacting, can cause even more pain.
There are, however, "inappropriate" emotions. For example, the Talmud forbids "crying too much" — i.e., for too long a time — at the loss of a loved one. Eventually a person must get over his loss and move forward.
Rabbi Yisrael Salanter is reputed to have said: "When a child's toy breaks, he feels as bad as an adult would feel if his factory were destroyed." In short, people experience losses at different levels, so it's nearly impossible to judge others' feelings.