Take a guess and think about what word I searched. What word caused that photo of an inspiring quote to pop up. Have you thought about it? It's one simple word; it is even shown in that quote. Life. 

I have never thought of life like a camera before, but hey, a new perspective is always a good one (before you argue against that, let me state that it forces you to take a step back, and think about things in various ways, which is always a great thing. Seeing how other people look at things can refresh your mind as well as give you a clear head to think again). Comparing life to a camera - focus, capture, develop - is a great comparison simply because it is true. That is how life needs to be lived. 

Focusing on the negatives is never a good thing. You forget about the positives that are happening in your life (or already happened). You ignore the important things that really demand your attention. If you are trying to live a joyful, good life, then you need to see what is in front of you and live for that. Why would you focus on things that you cannot change? That would a. waste your time, and b. take time away from enjoying the things that you have the power to make a change for/to.  I look at life like this: if you can't change it, don't give yourself migraines about it. Get it, got it; nod your head yes.

Capture the parts of life that make you smile, the parts that you want to look back on and say, "that was amazing." Forget about all troubles.  Live in the moment so that that moment will be forever ingrained in your head. Think about this: when you take photographs, you get the good and bad things, but what do you prefer? The good things, usually. So why not capture your life like that? 

Developing the bad parts are necessary to grow in life.  It allows you to learn, understand, and continue on.  The part that you need not get mixed up with evaluating whatever that bad thing was is focusing on the bad parts of life.  Do not focus! Do not orbit your entire life about something that happened and cannot be changed. You will get nowhere when you are like that. Instead, understand what happened, cry about it if you please, then move the hell on. Bad things happened to everyone, but it is up to you (and only you) to choose how you want to respond to that and what happens after. 

Besides, your life has plenty of film that is available, so if one shot comes out quite icky, you can always shoot as many more as you please. Don't waste it on invaluable things.