The speaker looks back at a string of failed relationships and tries to find the cause. Bad luck seems unlikely when the pattern repeats so often, and bad taste in women doesn't fit either. That leaves one thing they all share: him. He calls himself the "common denominator" and the "inevitable perpetrator," words that sound almost like a courtroom verdict.
What makes the poem sad is the gap between how much he gives and how little he gets back. He opens up, makes himself vulnerable, and is met with silence. The "stabbing pain in my chest" arrives as the woman leaves without any trouble, while he assumed she felt the same. He wants to blame them, but he turns the blame inward instead.
The mood is heavy and tired, a mix of anger, sadness, and loneliness. The short, plain lines read like a man talking himself through a conclusion he doesn't enjoy. The last line shows what lies underneath all the self-blame: he just wants someone to love him.