Spark or no spark?

I'm in a happy relationship and recently it was an old friends birthday so I sent him a Facebook message Saying 'happy birthday :)' he responded with a rather long message asking how I've been and added at the end 'miss you a bit :)' anyway we had a brief conversation. 
This friend and I had a thing for each other many years ago and I will always find him attractive because well he's pretty damn hot and every time we speak since he admitted that he also had a very big thing for me at the time that I did, he's always overly flirtatious or sweet I don't know and always mentions he misses me and all that and always says he wants me to be happy and all this. 
Now both of us are in relatively happy relationships, well I know that I am happy don't know about him. but anyway curiosity killed that cat as we all know and I'm always dying of curiosity whether every time we speak or see one another it sparks something in him too, because of the whole what if scenario. Don't get me wrong I wouldn't leave my other half for him because we have a life and house together now and I'm happy with that. But I'm curious about how he sees me. 
 Thoughts??

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