I heard somewhere that Billy Graham’s wife was asked by a journalist whether she always agreed with her husband. 

Her answer was, “if two people agreed on everything one of them wouldn’t be necessary!” 

It might be apocryphal, but it makes an important point - being in a close relationship with someone doesn't mean you always have to agree.
Perhaps the concept doesn't apply to the persons of the Trinity who agree on everything and are each absolutely necessary. But for we humans, at least during this present age, disagreements don’t equal bad relationships. On the contrary disagreements will necessarily be a feature of the very best relationships as together we help each other move closer to God’s truth.
Paul does call people to be ‘of one mind’ (Romans 15:5, Philippians 4:2) and it is very important that we agree with one another on the fundamentals of our Christian faith. But beyond those things that are gospel issues, our differences can help lead us to truth and express the fullness of God’s creation. 
Differences of opinion can help lead us into truth if, and only if, we express our differences and talk about them openly together. This is a huge challenge - there’s the challenge to express myself clearly as well as the challenge to understand you accurately and vice versa. A productive discussion about significant differences of opinion is going to take time, effort and a certain amount of skill to accomplish well. Above all, that sort of discussion requires grace - grace to listen carefully to others, grace to present your opinions and disagreements gently, and grace to assume good motives of each other rather than bad.
This means we need to have strong and deep relationships with one another to be able to discuss differences of opinion together in a productive way. Enemies will fight over their differences. Acquaintances will paper over their differences. Friends and family will argue through their differences and help lead each other into truth.
There are also other differences that are neither right nor wrong. Some differences are just part of our created otherness (like enjoying rubgy or chess, being an extravert or an introvert). 

Other differences have equally valid biblical foundations (like infant or believers’ baptism) that God hasn’t provided a clear answer on.

In most relationships we decide to band together with the people we agree with (and form a rugby club, a chess club, an infant baptist club) and separate from (or even demonise) those we disagree with. It takes the very best of relationships to be united by common fundamental truths while retaining an enthusiastic commitment to the things that are different between us. In doing that our common fellowship is enriched by each others’ difference rather than being diminished by its absence.
This unity in diversity is one of the glories of the church that has been called into being by the will of the trinitarian God. The Spirit of God binds us all into unity with Jesus the Son and in that unity we are being led into the fullness of truth in God the Father.
So, perhaps we should make a New Year's resolution: find a brother to disagree with! It's a godly thing to do.