In extremely rare circumstances will cats who are strangers immediately “like” and accept one another. In most cases, there is bound to be some animosity and because of this it is key that cats are introduced to each other in stages and not just left together to “get on with it”.
The best time to bring a new cat into your home is when there isn’t much else going on, for example doing the introduction at the same time that you have overnight visitors staying over would not be a good idea. The “resident” cat needs to be relaxed, and you need to have enough time to manage the introduction process.
Cats will always get on better if they don’t feel that they have to compete for your attention and affection, or even over their “resources” such as their bowls and litter trays.