Why do my mercenaries keep leaving me?

[Psitticine]

Side-note: When you "re-up" them, your bid will count double, so it is almost always cheaper to retain them for 3 more turns than it was to hire them in the first place.

[vanedor]

sigh,I don't know how many time I lost my mercenaries because I forgot to check the month they were at.

I wish there would be an automatic rehire feature.

[MythicalMino]

either that, or a message that says that some mercenaries are planning on leaving next month