There's no excuse to be bored. Sad, yes. Angry, yes. Depressed, yes. Crazy, yes. But there's no excuse for boredom, ever.
Human bodies are designed for regular physical activity. The sedentary nature of much of modern life probably plays a significant role in the epidemic incidence of depression today. Many studies show that depressed patients who stick to a regimen of aerobic exercise improve as much as those treated with medication.
Typically, when you have a depressed individual, they feel hopeless. They feel miserable. Their mind is racing, their heart is pounding. They feel anxious. They feel exhausted yet they can't sleep.
If you know people who are suicidal, or if you know people who are bipolar, depressed, have panic attack disorder, just be there for them. They're going through something that's very, very hard.
When an introvert is quiet, don't assume he is depressed, snobbish or socially deficient.