If you say "that sucks" the other person is going to assume you're agreeing with them that the thing they're angry about sucks. They're not going to think you're saying "that sucks" that they have an emotion, as an isolated feeling that happened for no reason.
This is where the overly academic concept of "validating emotions without endorsing them" falls apart in the real world.
In actual human interaction, people don't debate if the other person actual feels an emotion. Angry people don't need other people to agree that they feel angry. They share the emotion because they want other people to agree that the emotion is right and justified.
Nobody actually says "I agree that you are feeling that emotion but I neither endorse it nor disagree with it" (in less formal wording). If you're going along with someone else's emotions, you're implicitly endorsing their reaction as justified.