I think it’s because everyone has their own issues so when it comes to other people they either a.) find comfort in helping someone else with their issues or b.) would rather avoid the negativity all together and use your interaction as a distraction from their issues.
The second theory is that people just genuinely don’t have empathy or sympathy... some people just don’t know what to do or say when someones crying.. some people don’t know the right words to say when someone dies... so when bad things happen, you’re not always sure what to say or do so they prefer changing the subject or joking as a way of dealing with it. It comes off as insensitive but the person may genuinely not know how to handle the situation because they’ve never been there or because they just dont have the words. Theory three is that some people are just selfish. Ive had people in the past ask why im so negative all the time when i share my issues yet when they break up with their bf/gf, get in a fight with a family member, run into work or money troubles, guess who they pick up the phone and vent to? Me.....relationships are give and take no matter what kind of relationship and some people would rather take advice left and right but cant be bothered to offer you any comfort or support..... you just have to find the true souls who wont invalidate your feelings, who don’t dismiss you, who could hear you rant about the same things 100x and would still give you a hug. You deserve support. 100%