Hamsters often fight because they are territorial. Territorial fights occur when cages are too small or when a new hamster is placed in another hamster's cage. According to Hamster Fanciers, hamsters may also fight if they are different breeds and placed in the same cage.
Pregnant hamsters can become particularly aggressive toward male hamsters. Once a hamster becomes pregnant, she needs to be placed in her own cage away from other hamsters.
Hamsters do not always fight out of anger. In certain instances, two hamsters housed in the same cage may simply play fight. Paw Nation recommends housing hamsters in separate cages permanently if play fighting becomes too aggressive or violent. When hamsters begin fighting, it is a good idea to look for signs that the fight is escalating to aggressive fighting. Excessive squeaking or prolonged "play" fighting are both signs of aggressive play. Leaving two hamsters together while they are fighting can lead to severe injuries or death for both hamsters.
While certain hamster breeds such as Campbell's hamsters do well living together in pairs, many hamster breeds do not need to live together. According to Pet Hamster Care, Syrian hamsters and Chinese hamsters do well living alone. However, certain breeds such as Roborovski hamsters do well living in pairs or in multiples.