I try and give them a reason for doing it - character based, history based, and then try to lead the audience through that journey to that point.

Always a solid idea--nobody consciously decides they suddenly need to change their lifestyle or behavior without a reason.

And now I think you've got me considering this issue from a different angle: instead of trying to convince the audience, focus on convincing the character to change in-story, and if if the character is able to buy it, your audience should be able to as well.