Feeling down is not always a medical condition; if it's an adequate reaction to the conditions you're in, it's not something to be cured or treated. May be in your case, or in cases you're taking about those people felt depressed because they didn't have enough social interaction, and they fixed it by being around other people more: then it's not a condition, just your brain doing exactly what it's supposed to.

Depression, on the other hand, is feeling down without any adequate reason to do so. If your life is objectively quite okay, save for depression itself, but you constantly feel as if you've lost a loved one, then it's a medical condition. It's like feeling physical pain when you're not actually getting hurt.