A relationship that isn’t supportive is not really a relationship at all.
For many, one of the main benefits of having a romantic partner is that they make their life better in all possible aspects. They are there to support you during the hard times and to be the first to champion your successes when things are looking up. If they prove incapable or unwilling to do either of these things, it can leave somebody thinking whether their relationship is actually in their best interests.
It is an unfortunate reality that those in relationships often experience these kinds of disappointments, and they can become the grounds for major arguments. Sometimes a partner might have a good reason for giving the best support that they can, but more often than not, this kind of behavior reveals opinions and behavior patterns that are only likely to cause more problems as time goes on.