Nouns / phrases:
Relationship, interrelation, connection, link, tie, bond. Causal connection, especially: the cause of something. Bridge (literally but especially metaphorically speaking), joining.
Affiliation, liaison, association, alliance, union, fusion, syndicate, coalition.
Adhesiveness, cohesion, coherence. Emotional attachment. Social cohesion, group cohesion, solidarity.
Something which is shared (e.g. mutual interests); commonality.
Commitment, agreement, promise, pledge, contract. Engagement, marriage. Something which is morally or legally binding.
Reliability, dependability, predictability.
For some people also: repetition, circles, cycles.
To relate, link, connect, bind together. To bridge a gap. To cause. To affiliate, liaise, associate, fuse, unify. To commit, agree to, promise, sign a contract. To adhere, cohere, hold together. To be emotionally attached. To share, have in common. To propose, get engaged, marry. To rely on, count on.
Related, linked, connected, tied, bound together. Affiliated, associated, unified, fused. Bridged, joined. Caused (by), causal, for a reason. Adhesive, cohesive, coherent, solidly united, emotionally attached. Committed (to), stipulated. Engaged, married. Reliable, dependable, predictable.
As a person: Someone with the above qualities, e.g. someone committed, reliable, predictable. Someone we have something important in common with. Depending on context, someone who is emotionally attached to the querent, or the other way around. Traditionally often used: someone who is engaged, married.
Make a connection! Find the cause/the commonality! Bridge the gap! Stand united!