Peter can't say "I love you". He never did not to anyone, not his father, not Dana and certainly not to the other Ghostbusters.
Peter isn't a man that can express his feelings with words. Saying things means he has to show vulnerability and vulnerability means people will leave. People leaving means he is alone and Peter Venkman cannot be alone.
Instead Peter shows his affections with acts of services. He claims he doesn't care, he claims he finds some interest boring from the others or at least not that interesting. Romance movies? the same old. Spores, molds and fungus? blegh. Books about occult, spirits and more? that's basically all his job!
and yet Peter shows and does things he complains about. "not another episode of star trek" he would whine yet sat idly next to Egon as they watched their VHS tapes. "You talked about the spirit guide already" he complains yet listens when Ray goes on another passage, watching how Ray mouths moves. "The junkfood is meh" he would say sarcastically when Winston invited him to another take out place, with good food, because they all know Ray and Egon wouldn't taste new food unless the other 2 already did.
He goes shopping or in other occult bookstores to find mushrooms and books Ray and Egon likes. He looks at expensive watches for Winston. He memorised every order for every take out place from every buster with precise detail.
He knows what makes them calm down. When he sees something that reminds him of one of them he gets it and gifts them. He cooks if Winston is to tired and makes sure they all won't just eat junk food. He distracts them when they are in distress while making jokes.
In reality under all these snarks remarks and asshole-ish personality is a man who was deeply in love with 3 of his male friends, co-workers and businesspartners. Under that was a man who was deeply afraid of these men leaving him. A man who could not say what he wanted and everytime he tries his heart starts beating faster and he felt like he was getting chocked to death.
Peter Venkman was a man who was deeply afraid of comitment. He could not stand labels and being tied to another person and yet...
Peter Venkman was a man who would rather be caught dead than ever leaving Ray Stantz, Egon Spengler and Winston Zeddemore.