#2 Both players are currently unhappy
Admittedly, player unhappiness is a difficult topic nowadays. In this day and age, where tantrums are thrown over insufficient birthday congratulations and players are actively forcing their moves by not showing up to training, clubs can't budge at every show of discontent.
In this case, however, both teams could benefit from these circumstances, as the players are willing to cooperate to make the deal happen. While Sanchez is well-known to display his frustration on the pitch, Mourinho's problems with Mkhitaryan are also evident.
Assuming the move goes through, both clubs will hope to bring the absolute best out of their newly acquired players. The players, on the other hand, can start their next career steps with clean slates, something the Armenian playmaker could benefit from greatly.