The question was asked my someone while chatting today. I phrased the explanations then and there, but it turns out to be pretty correct. The reason I like ruins is very much like the reason I like Astronomy. Astronomy makes you realize how small you are in the spatial dimension and how vast things could be. Ruins make you realize the same in the time dimension.
The imagination that these ruins could have been inhabited by real people at some point of time gives me a high! I think my fascination for ruins arose when at some point of time in school, I read a description of ruins of Nalanda University. It was in our Hindi textbook – I do not remember the name of the author etc. But there was a paragraph in that piece where the author was trying to imagine what the place would have been like when it was actually inhabited by students and monks, what those rooms, the open spaces and graveyards would have been like, what they would have meant etc. This description had a strange effect one me! I also had a chance to visit those ruins when in school itself. Almost at the same time, my interest in history as a subject was increasing. The two effects combined brought me to a situation that as soon as I enter a ruin or an old building I quickly go in that imagination mode and that period is a period of bliss – only if you have experienced it would you know of it! You might experience it in a completely different thing, of course.
I do love ruins and old buildings!