Justin McClain's post on categorizing The 20 Types of People You'll Meet At Mass. That's why.
In the case of #16, you won't meet him - you'll just see him. Otherwise you ignore him - unless you are #9 - but you will still not speak to him.
I hope Mr. McClain didn't compose his post in his head during Mass. What?
I know the post is meant to be humorous, but it misses. Over the years, one of the most popular excuses I hear from family and friends as to why they don't go to church is summed up in McClain's post on the 20 types of people you will see at Mass. The most common excuse for avoiding Mass is this: "People are there to size one another up, see what they are wearing, talk about what they do or don't do, and categorize them." I know otherwise faithful Catholics who have skipped Mass because of something as silly as a bad hair day. People might talk if you don't fit 'Sunday best' requirements. Then there are those who back slap and shake hands at Mass but ignore you in the grocery store. What's up with that? But I digress.
So anyway. Go to Mass to pray, to worship - but stop eyeing your neighbor. In big cities, like New York or Chicago, or at any urban parish, things are more diverse, and people are less likely to get into this type of petty speculation.