When we reflect we quickly notice that much that happens in our lives is at best partly up to us. As long as I'm free to pick my surroundings, I'll select pleasant ones over dire ones; when it is open to me, I'll select being with interesting people who help me get along better over those of the boring, self-centered or deceitful kind; if I can choose, I'll select peace and stability over a life of strife and uncertainty — who wouldn't? But we're often not in a position to choose with respect to such external circumstances (external in the sense that they're outside our own control). Whatever depends on them is subject to risk and uncertainty.
The only thing we can choose, and in effect have to choose, are we ourselves; that is, we choose our own selves. (We cannot not choose these.) Still, 'choosing' here means setting a goal and then working for it — there is no free ride in these matters; choice doesn't mean here that you simply choose and automatically are guaranteed to receive. Yet if we choose here we can achieve those goals, and whether we do achieve them is in our own power. That's a deep difference between choices in matters of character, of our own selves, and choices in external things. And this means, among other things, that choosing carefully and well here is vital.
A choice in matters of your own self has a higher impact on the success of your life than any choice in externals. In a sense, it has a more direct impact, for how you exercise choices in external things depends on who you are, and which goals you have set for being the person that you should be; thus choices in external things are better or worse in how well they fit with those primary choices. And from this it should also be clear that choices in external things shouldn't dominate choices in matters of character. They're secondary. Doing your own thing is more important than whatever else may happen to you from the accidental circumstances around you.
However, doing your own thing isn't a license to be careless or arrogant with others and what they do. Remember that attitudes such as these are elements of your character too, and thus part of those primary choices that shape your character and, in effect, your life. And you don't want to have your character influenced by carelessness and arrogance. Moreover, there's another, deeper point here.
When we select, there normally is a positive gain. For example, when you can choose and select a pleasant environment for your day; say, you decide to spend it in a beautiful park just outside town; then there's an option that's more pleasant for you than other options, which is why you choose it. But of course the set of options often isn't fully random. It's no accident that there are beautiful parks to spend some time in. (And they must be kept in order, safe from animals, chemical pollution, criminals or whatever else might spoil it as a place of recreation, and so on.) Similarly, if you can choose being with people who inspire, or living in peace and prosperity, that will partly be because someone keeps up these options. Many of the positive gains which are there for you to select come from the work of other people; sharing in the fruits of their efforts constitutes an interpersonal relationship with them: you owe them a debt of gratitude. (Some other gains are just given by nature or the result of blind circumstance, and of course there's then no point in being thankful for those.) Gratefulness then is a counterpart to the freedom to select. As other attitudes, this one should also be a part of your character; it's a primary choice as well. Take care.