One of the most important keys I have discovered in the course of my life is to surround yourself with a good mix of dreamers and realists.

While I agree with you, I would also add that a person is not one or the other per se. It depends on the present relationship between two people. They balance each other out. The annoying habit of some good friends, for example, of always saying exactly the opposite of what you yourself consider important at the moment. For example, when you are enthusiastically dreaming about something, a good friend comes and says "Get your head back on straight." But if it is she who is dreaming, you take that role, don't you?

It's not ill will, but rather a kind of unconscious test that friends put on each other to see how far their enthusiasm for a dream actually extends and whether the full-bodied project is not just a pipe dream but will come true. The role of the sceptic is potentially inherent in everyone and we always take it on when we notice that in a relationship the other person is building castles in the air but seems far from building them on solid ground.
Where we are proven wrong (because you never know), it may have been scepticism or realistically posed questions that provoked the other person or pushed them forward. But where we actually just listened to the other person dreaming and the first objections already threw the other person off the track, we know that his ideas would not survive the thought stage.

The very fact that everyone can be both, dreamer and realist, is such a good thing about humans.

Though I would now come back and say that there are tendencies within characters and theses tendencies are likely to be built up depending on how the environment reacts to them. We can also manipulate it to push ourselves in certain directions.

Greetings from far :)