Often, it is a spontaneous act which one person commits, without prior plan, because he/she thinks it would make his/her mate happy. Often, the intuition is right. Take this balloon, for example. My husband's company had an event. He saw the balloon, thought of me, and brought it home as a surprise. I bet he had imagined the giant grin on my face and he was right. The giant, round thing made me so happy the moment I saw it.
Thank you, husband, for thinking of me. In doing so, you've brought me joy.