Thoughtful is an interesting word because it can mean someone who thinks a lot, and because of that isn't easily manipulated, and also someone who thinks about others a lot, acting with corresponding compassion and understanding.
Sometimes I'll sit by a waterfall and just watch the water cascade down, minute after minute, in what seems to be an endless flow. How, I marvel, can rivers flow, and flow, and flow, and not run out? Water is one of the world's greatest treasures.