Houseplants are important psychologically. They demand nothing from us other than water and light, yet they are a living thing whose existence depends on us. By requiring our care they allow us to shift our focus from ourselves to something else, but at a much lower concentration than pets or humans. They're useful for staving off depression. The hard part is when the depression wins.
Mine started about ten. I'd had a couple houseplants and an aquarium in my room in 5th grade; when we moved in sixth I took the opportunity to plan a large skylight (which I never got - my parents put them in two other rooms but) and hang up a couple 4' grow lamps. By the time I was sixteen I had maybe 25 pots of various foliage, two aquariums and a hand-me-down cage full of finches.
By the time I was seventeen my sister was stealing my shit to sell to her friends, my parents weren't interacting with me unless it was to give me shit, I was a full-blown exercise bulimic and I was trapped.
And I let it all die.
At one point I went away for four or five days and my parents broke into my room because they suspected there were things to be fed in there. They apologized when they realized everything was long since dead.
But they never wondered what the fuck happened, and they never did anything about it.
I like green, growing things. My own little Silent Running in the middle of the fucking New Mexico desert was my escape capsule. And the fact that I couldn't keep it alive still messes me up.
The fact that my parents never gave a fuck made me angry for a very, very long time.
I once had three mice. I named them Theodore, Buttercup, and The Holy Spirit. They were feeder mice meant to be fed to snakes. I had them for about 6 months and they helped with my depression. Mice are particularly hard to pet, and they started to hiss and bite at me whenever I went to feed them in the cage. This went on for awhile and on one particularly bad day I decided to kill them off. I rationalized that they were feeder mice that were going to die anyway. I took a half empty jar of peanut butter and put then waited awhile before I put the lid on it.
I woke up the next day feeling the absolute worst, and I was worried that my family would come in and notice that my mice were missing. I bought three more mice and pretended that everything was fine, but in the back of my mind I felt that there was an aura of fear left behind by the previous trio and these new mice knew that I was a mouse killer. I kept them until I left to find myself across the US and I couldn't find anyone to take care of them and I didn't have any friends with snakes so I decided to kill them in the same way, in a peanut butter jar.
I don't trust myself to take care of pets or plants right now. If you buy a succulent I'll buy a succulent.