Guinea pigs are herd animals and really shouldn’t be kept alone. Even if you’re the best owner in the world and handle them every day, they’ll still be lonely without a fellow guinea pig to talk to, so it’s really important to keep guinea pigs together.
Guinea pigs are herd animals and should be kept in groups of two or more
It’s often difficult to tell if a guinea pig is lonely, but instead of trying to guess whether or not your guinea pig is lonely, you should assume that they are if they are being kept on their own. Even if your guinea pig does not get on well with your other guinea pigs, that does not mean that it should be kept alone. What you can do in these circumstances is either make sure that your guinea pig house is big enough for them all to have their own spaces, or put up a bit of mesh between that guinea pig and your other guinea pigs to stop them fighting. That way, your grumpy guinea pig can still talk to its friends without any risk of one doing any harm to the other. You can always remove this mesh when you feel the time is right.