Cognitive psychologists agree with the Shakespeare quote in the title. They say that any situation or life event is neutral, not good, bad, depressing, or anger or stress provoking until we begin to attach our thoughts to it. In addition to the stacks of research that support this idea, it really makes common sense that if we think in depressing ways about things that we'll feel more depressed, or if we attach stressful or angry thoughts to situations that we'll feel stressed and upset.
Many people argue, however, that it is life circumstances that dictate a person's mood. They feel that if only they had less work, more money, better relationships, cars that didn't break down, nice weather, improved health, etc. that they would be happy. But even if mostly everything in a person's life was going as they wished and they felt happy, the power for their happiness is still in the circumstances and events in their life versus their own control. As soon as something doesn't go well or isn't what they wished, their mood is taken in some other direction than peace and happiness. Life for them is like riding in a sailboat without any way to guide it. They get blown whatever direction the wind moves.
When we become better at managing the thoughts that we attach to the different life circumstances that arise, we are taking back the ability to feel more the way we desire regardless of what has happened. It is not the situation that determines our mood but rather the way we talk ourself through that situation.
To illustrate this, pretend that you and three of your friends are flying away for a week to an awesome vacation destination. When you arrive there, you find out that your luggage has been lost and that you have nothing but a few things you carried on the plane. If this event dictated how your group felt, all four of you would feel the same way, but you don't.
One of your friends might react with thoughts like this. "Why does this always happen to me? Every time I try to do something it goes wrong. My trip is ruined. This is just horrible. I wish I could just go home." And of course this friend ends up feeling down and discouraged.
Another friend might respond by thinking these thoughts. "I knew those stupid idiots at baggage claim were going to mess things up. I hate this airline, they always screw something up. Why aren't there any competent people anymore that can do their job?!" And as this friend stomps off to yell at somebody, they feel quite angry!
Your third friend might think stress and anxiety provoking thoughts. "Oh no, what are we going to do now? I'm never going to be able to buy all the things I'm going to need down here. I had all my favorite clothes and now my outfits are going to look totally stupid. What if they never find our bags and we never see our things again?!"
You are learning the importance of the thoughts that you attach to situations, so you think, "Well, I'd like to have my suitcase, but it's not that big a deal. The airline will probably find it in a day or two and I don't need that much to get me through. We've got our room, were in an awesome place, and I'm going to have a blast on vacation even without my suitcase" And you are able to keep yourself fairly calm and upbeat.
There are multiple little and sometimes big life situations or events that happen in a day or week to which we attach our characteristic thoughts. Begin by simply catching and observing as often as possible the self-talk that you tend to put on these situations. Recognition of when you are engaging in your characteristic thinking patterns is an initial step toward being able to better change your self-talk to the calming, upbeat, reassuring thoughts that will lead to the more pervasive feelings of peace and happiness that you may desire.