There's a simple rule for choosing Point of View: which character is the most interesting stuff happening to? Interesting stuff could be internal eg a moment of realisation or external eg a car chase. However, the chances are in any one scene more than one character has interesting stuff going on and sometimes however it's not so clear about who to chose.
Take Othello. There are three main characters: Othello, Iago and Desdemona.
Othello is a brave and honourable man with a weakness - jealousy. He's going to be influenced by Iago into believing his faithful wife Desdemona is actually unfaithful. In a fit of jealousy he murders her. When Desdemona is vindicated, in remorse he kills himself.
Iago is a clever man who has been overlooked for promotion and therefore wants revenge on Othello. He persuades Othello that his wife has been unfaithful, driving Othello to murder. Afterwards, his own wife spills the beans that it was all a plot by Iago, and Iago is arrested and sentenced to death.
Desdemona is Othello's faithful wife. Accused of adultery she protests her innocence but still gets murdered by the jealous Othello.
So they all die in the end. (Hope that wasn't a plot spoiler for anyone...)
But who has the most going on? It's fairly obvious that Desdemona has very little to play with. Iago has a lot of interesting elements to play with - thwarted ambition, deviousness, manipulation, but as a baddie he's a bit one note. Othello on the other hand has a great character arc full of change from honour to jealousy to murder to remorse.
Which might explain why it's called Othello, rather than Iago. Shakespeare shows that the interesting stuff - internal or external - is all about the character who has the most change.