The reason it matters to me is that it affects my motivation to do the things that make me happy. I started having mood swings as a child, and it escalated to clinical depression in my early teens. As I grew up there was no shortage of 'helpful' people in my life telling me things like 'smile', 'just be happy' and (the worst) 'cheer up - it may never happen'. My mother's hairdresser was a particular offender. For his troubles he earned my famous death stare, which soon shut him up.
Some other 'helpful' people write books and articles advising depressed people to 'think about other people more than yourself'. They may even say depressed people are selfish or self-absorbed. Particularly with people who - like me - have attempted suicide, they may point to the pain caused to other people as a reason to stop being depressed and suicidal. I met one young woman who'd had the terrible experience just after a suicide attempt, when she was feeling very vulnerable, of having a psychiatrist yell at her for causing her family so much anguish. Needless to say, this didn't make her feel any less depressed!
So when depression starts intruding to my normal habitual happiness, I find it helpful to shut down the echoes of those 'helpful' voices from the past. It's not that I don't care about how my depression affects others. I do. It's just that I feel so resentful and resistant in the face of these thoughts - just like I did when I was growing up - that it makes me act against my best interests and not do the things I know will make me feel happy.
What works better is to focus on the main reason I want to feel happy - because it makes me feel good. The fact that it is also nicer for others is great, but it's the icing on the cake. The cake is my own good feelings. I've spent too many minutes and hours of my life feeling painful emotions, enviously watching happy people and wishing I was like them. I treasure every moment of happiness I experience, and the reason I work so hard to achieve and maintain it is because it's so pleasurable. So when I feel down I say to myself 'I don't 'have' to feel happy - I want to feel happy for me, because it feels good'. This may seem weird, that I might resist doing something that feels yummy - like resisting eating chocolate! But if you have experienced depression you can probably relate to what I'm saying. Those 'helpful' comments can make you dig your toes in and refuse to do anything, even though you are the one who suffers most as a result. So focusing on feeling happy for yourself can be a way to get past that resistance. And everyone benefits!