If you or someone you know is struggling, looking on the bright side may not be helpful. Trying to reason away negative feelings or demanding that you or the other "Snap out of it!" likely won't do any good and may, in fact, be harmful.
Instead, when someone expresses their emotions, listen, then acknowledge and validate their feelings. Instead of assuring them that everything will be OK, say, "I'm here for you, I care for you," and ask how you can support them.
If someone feels overwhelmed, give them space to talk about their experience without judgment. You can say, "That sounds hard for you, tell me more about that."
Don't tell them what you would do or feel in their place. Instead, ask them if they just need to vent or want advice.
If the negative feelings deepen or linger, don't play therapist. People with persistent depressive feelings should be encouraged to see a physician and a mental health professional.