Self-esteem.

Have you ever had a bad day? Maybe you woke up in the morning and just knew it would be one of those days.

Maybe you got into a fight with your partner or friend, or someone cut you off in traffic. You know that feeling when you're sitting at your desk, dreading going home because of something that happened that day?

If so, then you already know how important self-esteem is. When our self-esteem is low, we tend to be more influenced by the desire to avoid pain than by experiencing joy. Negatives have more power over us than positives. With high self-esteem, we're less vulnerable to disease, are better equipped to cope, and can pick ourselves up after a failure.

So what's the secret to building self-esteem? Start small: tell yourself, "I'm proud of myself" if you accomplish something great (or even if not). It might feel silly at first, but keep doing it until it starts feeling natural—then go bigger!