1) The reason is that achieving agreement requires people to accept the reality of views different from their own and to accept change or compromise.
2) It is not just a matter of putting forward a set of facts and expecting the other person immediately to accept the logic of the exposition.
3) People need time to make this adjustment in attitude and react badly to any attempt to rush them into an agreement.
4) They (and probably you) have to be persuaded and helped to feel comfortable about the outcome that is eventually agreed.
5) In general, there is a tendency to underestimate how long it takes to discuss and resolve an issue on which two people initially have different views.
