So the question of motives… Let’s start with motives in others.
We find them – or believe we do – from observation and inferences. We are mostly sure about them, some people are, ordinary people mostly are. Forgetting a birthday or an appointment, a name, or distorting a name, are instantly put down to some motive: Loss of love, lack of interest, desire to hurt the other, or whatever it is.
These motives are not directly observed though. They are inferred!, and not always with precision.