A backstory in the first season showed that the Digimon partner's eggs were not entirely natural (compared to other Digimon) and had some 'science' modification that imprinted them to the human partners even before meeting face to face. One of the Digimon partner's eggs was dropped during an evacuation of the lab they were made in and hence hatched separately from the other seven. After that partner hatched she spent time wondering, feeling like she should be awaiting somebody but not knowing who and hence feeling empty/ unhappy because of it. EDIT: Also season 2 established that partner Digimon can weaken if separated from their human partner. One character had a field trip to Kyoto and left her Digimon partner with another of the main characters to take care of for that duration. The result was that Digimon's health deteriorated requiring the temporary caretaker to travel to Kyoto via the digital world to reunite the Digimon and human partner.

Season 3 (and I think 5) was when the Digimon partners were not born wanting to have a human partner and could choose their human partner. One Digimon's backstory in 3 was that his human partners (who were much younger kids) were not good partners which got him to run away and the emotional consequences.

spoilers for season 3

Some time later in season 3 they mentioned that Digimon were made to have some affinity with children in general. So still kind of conditioned/ brainwashed like season 1 and 2 but not as bad.

4 had no partners; the humans became the Digimon themselves.
I don't know about the later seasons.
EDIT 2: Added some more information for season 1 and 3 and put some plot-relevant stuff behind spoilers.