Call a person sentimental if he or she is led more by emotions than by reason. If you have a sentimental attachment to a favorite stuffed animal, you'll probably even bring it to college with you.
Sentimental describes a person who relies on emotions more than reason, or a novel or film that is overly emotional. Derived from the Latin verb, sentire "to feel," sentimental always implies that the emotions are involved a little too much. If you do something for sentimental reasons, you are only doing it because of an emotional attachment.