RunnerBabe: For some reason a lot of men equate the male chest with being manly and virile (seeking sex). This in turn, on a primitive level, will attract women. Like the male rooster puffing out his chest, seeking a mate. Of course we all know that a roll of hundred dollar bills, flashed in a bar, in front of women, will do the same thing now-a-days. (getting sex)
Same thing could be said about the males arms. Muscular arms are supposed to indicate power. And again, trying to establish a manly & virile image (seeking sex). A man in charge. Of course we all know this to be a delusioned state of the male ego. The only thing that muscular arms will suggest is that he has two of them. One on the right side of his body. And one on the left side of his body.
Lat's also fall into this catalog of maleness. A flair of the lat's give a dominate image of size, control and sexual power.
If you have ever read The Naked Ape (Dennis..and other works) it may become clearer why people act as they do. So now we have a clue to why your training partner worked chest so much. And why thousands of average, skinny men will just do light pec's, arms and lat pull's exercises only, when they workout at home or a gym. Good Luck.