From what I've read, attachment to another person is just as bad (think unrequited love, or even a co-dependent relationship). The thought goes that you essentially give the other person power over your happiness, so it's bad for you.
It's also not good for the other person either. For instance a scenario where your girlfriend may want to go to culinary school which is best for her, but if placed all your happiness in her, then you may be tempted to sabotage her attempts to follow her dreams (to varying degrees), so that she may stay with you.
These are at least the thoughts of an interesting fellow who was a jesuit who later embraced buddhism and was found to be "incompatible with the catholic faith", so you know it must be pretty good stuff: https://www.goodreads.com/author/quotes/54195.Anthony_de_Mel...
It's also not good for the other person either. For instance a scenario where your girlfriend may want to go to culinary school which is best for her, but if placed all your happiness in her, then you may be tempted to sabotage her attempts to follow her dreams (to varying degrees), so that she may stay with you.
These are at least the thoughts of an interesting fellow who was a jesuit who later embraced buddhism and was found to be "incompatible with the catholic faith", so you know it must be pretty good stuff: https://www.goodreads.com/author/quotes/54195.Anthony_de_Mel...