Romans 15:2

‘Let each one of us please his neighbour unto the good, resulting in edifying.'

And the aim behind this is the pleasing of our neighbour in order to achieve ‘the good'. That does not mean putting the pleasing of our neighbour before our pleasing God. Indeed, the point is that by achieving ‘the good' we will be pleasing God, for the idea behind the good is of what God sees as good. The good includes the good result of sustaining the weaker brothers and sisters, but probably also includes the final good resulting on the widest scale from obeying what had become Christ's commandment based on Leviticus 19:10, to ‘love your neighbour as yourself'. By loving one another we sustain one another.

The use of the term ‘neighbour' rather than ‘brother' clearly suggests that Paul wants them to see their attitude as in line with ‘loving their neighbour' (in the New Testament the use of the word neighbour is almost always in that context). That in this context ‘the neighbour' is a fellow-Christian is apparent from the fact that pleasing him will result in edifying, that is, in his being built up in the faith.