Attaching oneself without the ability to let go is my thought. Attachment in and of itself is good. However attachment without wanting to let go is a selfish act. I think selfishness would be a better choice of words. Selfishness may produce temporary happiness from a happening, but in the end produces unhappiness because it is not fulfilling. You are always wanting something else or something more. Selfless attachment is true love and will find joy in the giving away of that love.