Don't leave them when they need you the most.
Depression is a serious mental illness that many people struggle with in their everyday lives. More often than not, it disrupts the way we feel and act in many situations - work, eating, relationships, showering, friendships, etc. It is not a joke and people who struggle with it need to know that someone is there for them.
If you are dating someone who happens to be diagnosed with depression, help them cope. This is not to say that you should be trapped in a relationship to keep someone happy, but a warning that if you love someone who is struggling, be there for them before it is too late.
Depression can come in many forms because people who are diagnosed cope with it differently. Sometimes it is present in not finding the motivation to shower for long periods of time. Other times it could be skipping classes or work because you physically can't get yourself out of bed. One day you may binge on everything in your kitchen, while unintentionally starving the next.
It is a constant battle, even when it is suppressed and feels minor. Some days are better than others, but it is definitely not an easy thing to live with.
If your boyfriend, girlfriend, family member, or even your friend is struggling to fight depression, be there for them. You do not have to do a lot, but provide an ear if they need to talk it out. Allow space if they feel constricted, be clingy when they feel alone, give them reassurance even if they've asked 20 times in the last five minutes. These are the days when they need someone the most.
Do not leave someone to fight depression alone simply because you can't stand that they are "sad all the time" or they "don't do anything fun" with you. They need you. They need help.
Often times hints go unnoticed and people with depression are passed off as lazy, selfish, boring, and downers. Sometimes people ignore the hints until it is too late and they cannot get their loved ones back.
Sometimes you cannot tell how someone is feeling, but there are subtle hints. Please pick up on them and show them that they matter to you.