Well, in my experience (I have multiples of all my animals and the only ones that stay 'by themselves' are the male canaries but that's only because they belong to a territorial species and not a social one), it's actually better for the human for the bird to be happy (because happy means less stress -captivity for undomesticated species ALWAYS means stress and that's not my opinion, it's a cold fact of nature- and less stress means stronger immune system which, in turns, means a longer life) and I've never 'lost' an animal's affection just because it happened to have a companion or mate . Quite the contrary, I always end up with two that love me instead of just one. But, in all honesty, I am an animal lover so I would never consider putting my desires before an animal's needs so, if a species is social, I will strive to give it, at the very least, a mate just for the simple reason that I want my animal to be happy with me. And, no, male lovebirds never fight with females or other male lovebirds. Two females that have no mates of their own, are housed together in a crowded condition, kept at the wrong light schedule and fed the wrong food would though - it's a matter of knowing what to do. And their having babies is also just a matter of knowing what to do. I have males and females of several species living together but I never get babies so, in reality, EVERYTHING is just a matter of the right husbandry.
My position when it comes to parrot care is to follow nature. Period. People's opinions mean nothing to me. Not because I think I know more than them but because I know I don't know enough and neither do anybody else out there (there are no parrot experts, not yet anyway, because the pool of knowledge we have is very small and there haven't been enough years of keeping parrots as pets for any relevant accumulation of compared notes - we do with canaries, for example). If the species is social, I keep it with others of the same species, if the species is territorial, I make sure each male has its own 'territory'. Mother Nature is the only one that knows enough so all I do is follow her guidelines.. and it has worked like a charm for me for many years and with hundreds of birds so, no, the 'if the bird has a mate, you lose your friend' is not true. And I know for a fact that it's not true with lovebirds because I had a female called Matilda (she was an Australian cinnamon) and this bird did not only even left her nest with eggs she was incubating (the eggs were rendered infertile) just so she could come and spend time with me, she even taught her mates (she had more than one but not at the same time) to come to me when I called her (this is because they are always together and the male follows the female wherever she goes). People will tell you something they read somewhere as if it was the gospel even when they did not experience it themselves or done any real research on the subject. I remember many years ago when I first took in my first group of lovebirds. There were nine of them, all show and breeding birds (back then, they were called 'cherry heads'), and they had all been kept separate their entire lives (because they also believed back then that they would fight) and, when I asked (also in a lovebird forum) if I could make a colony out of them, EVERYBODY said -and very categorically, I might add!- that it could not be done. But this made no sense to me because these birds live in flocks in the wild so, OBVIOUSLY, a flock was their natural social grouping (and this is hard-wired into their genes). I let out the pairs one by one and they did not only NOT fight, they immediately formed a little flock and lived happily ever after