The Slot Machine
You're at the casino and decide to play the slot machine. You put in a few coins, pull down the lever, and hope you win the jackpot each time, but you don't. You keep putting in coin, after coin, after coin. Every now and then you'll win a few bucks, but never the jackpot. But, hey! You'll take what you can get right? Despite making a few bucks, you keep praying for that jackpot. Who doesn't?
This is exactly what it's like to have a loved one that suffers from depression. You put in all of this work, all of this effort into this one person to make them happy, and you only get so much out of them. Every once in a while, you'll get the jackpot and they have a great day where they are 100% them; happy, carefree, loving life, but the next day you have to put in another coin, and hope for the best that you win the jackpot and get to see the smile on their face.
You give so many of your coins, but you only have so many coins for yourself. Sometimes you run out of those coins and have nothing left to give that person till the sun rises the next day. No matter how many coins you put in, levers you pull down, you don't get a single dime from the slot machine. You leave sad, feeling worthless, and as though you had accomplished nothing to help that person. You put all of your hard earned money into them, and end up getting nothing in return. You aren't upset with them, but you're disappointed in yourself, and that no matter how many coins you put in, you couldn't make them any better. So you try again tomorrow and hope for the best.
Tomorrow rolls by and you have ten coins for the slot machine. You pull the lever and get a few bucks! Lucky you! You'll take it! It's SOME type of happiness for that person, right? The player is rewarded because they're happy to have gotten at least a little something from the slot machine, and the slot machine is happy because.... it's simply.... happy.
Every day you put in the few amount of coins you have into this one person because you care so much for them, but when you are low on coins yourself, that person doesn't have any coins to give to you. You are okay with that and understanding because they are the slot machine, and you are it's coin provider. You are there for them and there to play along with their game. The slot machine and it's player.