If it is yourself:

Realize it is likely you feel this way for a reason, and it is a sign of health. Do an inventory of the main pieces of your life and note which situations are causing you pain:

- work life

- living/housing situation

- physical health (including movement, sunlight, eating habits and sleep)

- general social life (do you have spontaneous positive interactions with acquaintances, coworkers, baristas etc.)

- personal social life (relationships with partners/family and close friends)

If you are feeling depressed I can bet you more than one thing on this list can deserve attention. Pick one you can change for the better immediately and work on it. For instance, if you are lacking social interaction in general, go pick up a coffee someplace that is not too busy and either smile and make brief small talk with someone working there or another customer. If you don't like your apartment, put effort into finding a new one. Talk to your partner or get relationship counseling. Things are not going to magically become perfect overnight, but small things every day add up over time.

If it is someone you love: Listen if they are talking, talk to them if they are quiet. Generally be supportive. Search for a positive thing they are doing or want to do and give them positive reinforcement for it. They are stuck in a negative thought cycle and try to encourage something in them that is working. Here is an example: Friend: "my life sucks. I hate my job, my expenses are too much for what I make, and on top of all that, I just found out that my girlfriend is selling herself to strangers on Craigslist. I already confronted her about it, but she just says to accept her entrepreneurial polyamory and get over it. At this rate, I am never going to get a phd in deep learning or do a startup or have a stable family like I want"

You: "cool you want to get a phd? yeah I've heard about deep learning it sounds cool... which programs would you want to do/ what projects do you want to work on etc."

Remember that scene in fight club where Durden orders the clerk to follow his dream of going to vet school? Something like that. Listen to them to figure out where they want to be headed, and encourage that.