How we change what others think, feel, believe and do
Get others to comply with your requests by asking them to justify why they would not do so (and then showing that those justifications are not valid).
If people disagree with you, ask why they are disagreeing. Probe for the real reason, and then challenge this. Show how their justification for disagreement does not make sense or can otherwise be dismissed.
Say this assertively as if they will not challenge you.
You say you have to go to see James tomorrow. But you can see him any day. This is the only chance for a long time to see your grandparents.
Let's do it. Why not?
I don't see any reason for staying here, do you?
When you propose something, people may not be ready to challenge you. Their inability to offer a counter-reason can, in such situations, be reframed as agreement.
The way that you say your 'why not' can have a lot of influence in itself. When you speak with confidence, you challenge the other person to be more confident than you appear. If they do not have that confidence then they may well not take the matter further and just agree with you.
Asking 'Why not' positions yourself as judge and them as petitioner. If they cannot say why not then you are legitimized by this positioning to make a compensatory decision.
Why Not? is the 63rd of the 64 compliance-gaining strategies described by Kellerman and Cole.
Kellermann, K. & Cole, T. (1994). Classifying compliance gaining messages: Taxonomic disorder and strategic confusion. Communication Theory, 1, 3-60