I interpret the "hanging from a shelf" line as in it breaks his heart to see her 'up for grabs', so to speak, instead of with him, as in she is on display in a store or something. He can't stand to think of her in love with someone else so obviously he doesn't like the idea of other guys thinking about having a relationship with her. Since she won't go out with him, she is technically 'on disply' or 'back on the market' or whatever the phrase is. It has NOTHING to do with suicide or hanging somebody.
I interpret the "hanging from a shelf" line as in it breaks his heart to see her 'up for grabs', so to speak, instead of with him, as in she is on display in a store or something. He can't stand to think of her in love with someone else so obviously he doesn't like the idea of other guys thinking about having a relationship with her. Since she won't go out with him, she is technically 'on disply' or 'back on the market' or whatever the phrase is. It has NOTHING to do with suicide or hanging somebody.