Is it normal to have very occasional thoughts about an old boyfriend?

Im happily married for almost 10 years. A couple of years before i met my husband i had an on again/off again relationship with someone that i never really got complete closure with. We remained somewhat friends but only talked/emailed once or twice every few years.. He did come to my wedding though. Now i havent seen this man in almost 10 years but i stll occasionally hear from him and i immediately think back to those days, and sometimed have an urge to have lunch/catch up, etc. And i find myself thinking about him often. After a week or so he starts to fade from my mind and i go back to my normal routine and dont think about him much if any.
Let me be clear.. I will not cheat on my husband and have been very open with him about this old boyfriend and his ability to 'yank me back to the past very quick'. My concern is more about why does he seem to have this strange hold over me? Has anyone else found themselves in a similiar situation? Is it something i should be overly concerned about? considering it only seems to be an issue for a week or so every few years when i suddenly hear from him.
Would appreciate any help/insights on this.