The problem is that one day you will end up working in a company where pair programming is mandatory.
You might be stuck sitting next to someone with a poor hygiene for the whole day.
By your 40s you might get a back pain because you had to seat in an uncomfortable position.
You would not be able to work at the time you want to work (it is strictly 9–5, as you both need to be there at the same time) — so when you thinking of been able to collect your future kids from school, forget it.
You will be regarded as easily replaceable (just put someone else on that “pair”).
You won’t be able to use tools of your choice (everybody should use the same tools to be able to work together).
Don’t complain about it when it comes as you are supporting this idea now.