Well I don't know if you can actually talk about depression in this case, but you know sometimes you just feel bad, like not for a specific reason but you just feel bad, so you just go ahead and you don't tell anybody because you know you won't be taken seriously like you didn't have a breakup or got friendzoned, you are not poor, you have friends (and even trustworthy ones), but you still feel something empty in your life that you just can't fill whatever you do. I think everybody ever felt it, but just sometimes it lasts for a very long time, even years, and you just still seem to be "happy" to the eyes of others, I mean you try to look like but inside you're still feeling that void. Even sometimes when you go home you have to do your homeworks you just stand at your desk watching right in front and just losing yourself in your mind. The problem when you feel like that is that, from a certain point you start devaluating yourself and you start to see only negative things, and as time passes you start to have some depressing thoughts and there you start to realize you are depressed, I personally experienced it for years, when I was younger, I'm still a teenager tho, but I think it's pretty common actually and it's kinda rare to talk about it but sometimes people can be depressed even if there are no "apparent reasons".