PDA around me makes me draw a little closer to my S.O. and stirs up passions that may have become dormant in this busy life.
It is kind of like going to a wedding. I look into my S.O.‘s eyes and she looks in mine and it might as well be our wedding, and later it might as well be our honeymoon. I remember how vibrant, beautiful and young she was and is. She remembers how handsome I looked in my dress blues. <good shiver>
I think hanging out with young couples who are appropriately affectionate is good. Of course as some has answered, if they are all over each other and there is panting and groping, then I would laughingly tell them to go get a room.
I also recognize that if a couple are falling in love and appropriately affectionate and I am fighting with my S.O. then it makes me think of what actions I should do to restore our relationship so that it can be fresh like them again.
Perhaps if one is lonely or going through a tough time in a relationship see a young couple falling in love would prompt jealousy at some level.