It is the stability of the cat-human relationship which is so great
Companion cat advocates say that their cat provides unconditional love. By implication they are hinting that humans can’t. There is something in that but I think in all fairness to humans it is being a bit unkind.
Humans tend to lean towards being egocentric. They can be incredibly self-centred and self-indulgent. They do things for the benefit that it brings them which means that their love is often conditional upon getting something in return.

But cats are the same. Why should they be different? They are mammals like humans and they have emotions. They struggle to survive like any other creature on this great planet that humans are abusing. Cats receive food, warmth and security in return for their companionship.
What separates domestic cats and humans in the sphere of ‘love’ is their reliability. They are categorically there for you… except when they disappear! Seriously though, in general, domestic cats are more reliable than humans. The relationship is more stable than a human-human relationship.
The reason is that there is no game playing because of emotional responses. There are no hurt feelings with domestic cats. There is no tricky-dicky game playing and snide remarks. They don’t read into your actions something that is not happening.
I was going to say that they don’t two-time you and have someone else on the side but that is not quite true. A lot of indoor/outdoor cats form relationships with the neighbours and may spend time in their homes. But cats don’t do this out of spitefulness or because they are being tricky. They just prefer the other person’s food!
Why are there a lot of old ladies with cats? Well the answer is simple. They are not crazy cat ladies. They are sensible ladies who know that cats are very reliable companions supporting a stable relationship which is something they can no longer achieve with a person.
My conclusion is that on average the human-to-cat relationship is more stable and enduring than the human-human one. And on their passing – and cats generally die before us – often the human cries and grieves more than they did for their long lost human companion.