I just read a psychology book about this. From the book:

Internalizers are extremely sensitive and far more than most people, notice everything. They react to life as if they were an emotional tuning fork, picking up and resonating with vibrations from other people and the world around them. This perceptiveness can be both a blessing and a curse. As one client described, “My brain absorbs everything! I can’t believe how much stuff I pick up on, it seeps right into me.”