Things that preoccupy you engross or enthrall you — they suck up all of your attention and energy, sometimes to an unhealthy degree. Your upcoming math test might preoccupy you, making it difficult for you to concentrate in English class.
For some teenagers, thoughts about how they appear to others can easily preoccupy them, while others let their concern about grades and college applications preoccupy them. It's important to spend some time thinking about these things, but allowing them to preoccupy you can be stressful and unpleasant. Preoccupy comes from the Latin word praeoccupare, "seize beforehand."