i also can understand why they do it as well
if the op wants to have that middle seat between himself and his wife then he really should be booking the 3rd seat as well .
Plus size people have to pay for a second seat if needing the space
i think its selfish attitude to be thinking hmm if i book a window and an aisle for the wife and i , in the grand hope that the middle seat doesnt get taken
i do accept these things after all we should get to sit where we like providing the seats are not taken and you do't mind someone sitting in between ...
i do hope the origional poster is able to select the seats they wish for.