I am new to keeping ducks myself but have made some observations and done much research before I got our ducks. We have 6 ducks about 9months old that have been raised together since hatch day. Originally we had 8 but lost 2 to a raccoon, in separate incidents. We were heartbroken, our remaining ducks, who were present for the abductions and must have been terrified at the time, didn't seem the least bit bothered by the losses.
Ducks are social animals and we find they don't do anything separately- all actions are conducted collectively. One gets up to go forage, they all get up and go, too. One wanders to a shady spot and nestles in to nap and they all follow suit. Very social birds, much happier in a group.
That said, ducks do not mate for life like geese. In the wild you will see geese parents raising their young together and will continue to raise goslings with that same mate. Ducks on the other hand will only bond for a season and you will see the female duck raising her ducklings alone. The male will find another mate the following year during mating season.
They only form temporary attachments that should make them better adjusted if a loss occurs. That said, don't leave them alone. They are happier and feel more secure with another duck. Hope that's helpful.