spoiler
IM NOT LIKING HOW DEAN SHOOTING US HAS NOT BEEN BROUGHT UP FOR SO LONG....im literally screaming dont bring him along...how does dave not remember ??!
Spoiler and speculations
Because there are also "vision" aka iteration with Dom being friendly and Dean already being Dave's boyfriend or even fience. So although the Gun in Deans hand should make Dave suspicious, Dave is still too naive towards his friend. Otherwise he would have solved Dean's, Orlandos and Bensons murder case quit earlier.
Dean is suspect Number 1 on most murder cases (kinda framed by the master, like the hunting Knife in Hoss' chest or the big revolver in Bensons left hand, with Dean being the only left-handed person in the group).
Considering that Dean's Route is "unwanted" by the master, and filled with the master's mistakes to kill Dean at any cost -> I got the following hypothesis: That the outcome (and survival of certain characters) may even depend on Dean being romanced, Loyal or not. Or the opposite from the master's perspective instead the players: that Dean must die first (day10 - Route c/d/e).
I speculate that Dean has always been blackmailed by the Mafia to spy on Dave (and Orlando). Thanatos or his master already knew it all the time. Just as they knew that not making the trip to the mansion and David senior being alive will always lead to the death of everyone. So the vacation on the mansion is not only to "awaken" Dave's morphic resonance powers but also to shift Dean's loyalty.