I've learned myself that true friendships require investment. This doesn't always mean money, but it does require a lot of your time. Truly happy people spend time (and money) on their relationships: Giftology book and gifting expert John Ruhlin has written about this topic.

Research shows that feeling "rushed" can lead to stress and unhappiness. At the same time, people struggle with finding that happy medium of being just busy enough. After all, other studies suggest that a healthy work-life balance is key, since boredom can be burdensome.

If you're over-committed or too scattered, follow this rule entrepreneur Derek Sivers included on his site: "When deciding whether to do something, if you feel anything less than 'Wow! That would be amazing! Absolutely! Hell yeah!' -- then say 'no.'"