Someone who is interested makes real effort to get to know you as a person: asks personal questions about you, your work, your family or your day. He or she remembers things you say and let’s you know that they do.
The one who is not interested may keep in contact but asks no personal questions about you and avoids any personal questions about themselves. If he or she is lonely and just enjoying the attention, you find yourself in the role of friend/social support or therapist. Once they find someone else to lean on, they move on without any guilt or shame about using you.
But even for someone who is interested, if you make it too hard to reach you, to get to know you or spend time with you, the interest will fade out very quickly.
In other words, don’t be too elusive, too closed off and too busy that the other person assumes you’re not interested.