Romans 15:32

‘That I may come to you in joy through the will of God, and together with you find rest.'

And part of the reason for his prayer is that once those hurdles have been overcome he may be able to come to the Roman Christians with joy through the will of God (which will be determined by whether God answers their prayers), and together with them ‘find rest'. For Paul life had been a constant struggle with the burden of all the churches, and at this current time apprehension as to what might happen at Jerusalem. He hopes to find some relief from this during his stay in Rome, prior to further exertions in Spain. He would in fact find that rest, but in a way totally different from what he expected, when he lived in his own hired house in Rome under guard (Acts 28:30).