Their disease starts to drift them away. They are distant, they avoid social situations. They are practically paralyzed, living their life in closed doors. It's near impossible to get them to speak or to hold a "normal" conversation. They withdraw. And so, you begin to withdraw from them. Maybe you assume that they "need space".. Or maybe you think that they are just being "selfish" or "acting weird", so you're just gonna take a step back and let them figure it out on their own.
Don't do that.
Even if they don't admit it, a depressed person needs you. You don't have to follow them around or constantly ask if they are "okay" (most depressed people don't really know how to answer that question. They are niether okay or not okay. They are just numb). But what they do need is to know you care, and to know that you are still there. They want you to recognize their sadness... To be empathetic. Don't ignore what is happening but don't celebrate it either. Just be there. Make offers, knowing that if they decline it is not a personal attack... They simply aren't up to it. Make sure they eat.... Fix them a cup of tea. Tuck them in. Send them an encouraging song. Suggest that they seek help. If they are seeking help, ask how it's going. Just be there.
I know in these situations we sometimes feel helpless. I also know it's easier to ignore it, to walk away. But you have to fight. The person you love is still in there, behind the sad eyes. Look deep and embrace them for who they are and what they are going through. If you make it clear that you are standing next to them through this battle, I guarantee it will give them a glimmer of hope.
I am not an expert and I don't claim to be. But I was once in the battlefield. A few of my fellow comrads walked away and left me, they thought I was weak and couldn't understand why it was so hard for me to trudge on. They were wrong. I know this because I won the battle, with a few very strong and loyal soldiers by my side. Those soldiers carried me when I fell, pushed me when I needed it, and cheered for victory when I won.
Be a solider.