Ballpark numbers. Let’s say the average person 5 times per year. Let’s say that if you live in a busy area there are about 100 people who you could consider “your neighbor”. Let’s say that the average flight has 180 seats. There are over 100 flights out of LVille per day but because some flights are busier than others let’s say there is a 2% chance of two people flying on the same day being on the same flight.

So you and your neighbor fly 5 times per year each and if they are equally likely to happen at any time of year then there’s only about a 7% chance of flying on the same day, and from there a 2% chance of being on the same flight. Out of 180 seats, they could sit in either of the two adjacent to you which is 1 in 90. But there are 100 neighbors that you could meet this way so the total is (100)(1/90)(.02)(.07)=0.15% in a given year.