I can't agree with this. By your definition, "art" is anything we have strong feelings about, or things we react to. I have strong feelings about the city, and one of my past times is just walking around the admiring the flow of traffic and people. So my questions are as follows:
1) Would a city be considered "art"?
2) If a city can be considered "art", can't anything?
3) What is the purpose of a building being called "The Museum of Modern Art", or "The Metropolitan Museum of Art", if "art" not only refers to everything contained in those buildings, but the buildings around those buildings, the people who walk in and out of those buildings, and so on and so forth?
It seems the only way your definition would hold is if the word "art", as used in the names of these buildings, is a deception, that the buildings are implying boundaries where they do not exist.
Yet people still visit them, still pay admission for them, still consider everything inside "art", but everything outside "not art". How can this be? It's as though you can turn anything into "art" by taking them into one of these buildings and mounting a label beside it. Is that all it takes to make something "art'?
TL;DR: "Art is what someone considers art" is the most useless definition I've ever heard of.