Is it an illness for a person to be obsessed with another person?
No, it is not an illness, but it can be a symptom of emotional problems. It is one thing to be in love and “need” the other person, but it is another to be too needy, too clingy, to desperately dependent. And this desperateness can turn the other person off. Some obsession is merely a teen or preteen aspect of growing up romantically and sexually. If it’s an adult that obsessed, that can be an indication of problems–low self-esteem, insecurity. In an ideal relationship, or even a healthy one, both persons need each other, but each has his or her personal strengths, so then the neediness doesn’t become excessive or lead to slavery or disastrous disappointment.
