It takes connection to form an alliance.

Often we feel attachment first, we are attracted to the other person because of their listening skills, their attention to us in a quieter environment than usual (I have 4 siblings and I'm the youngest; getting attention in that household was not easy!) or our perceptions of their personal attributes, abilities, skills, etc. help us attach.

Think space station? Once attached, a connection is formed; the air lock is secured and doors opened so one can go freely from one side to the other. But just being connected does not mean anything useful will happen, that's the alliance; where the people from the shuttle decide to come aboard the station and run experiments and collaborate with the people already there.

Now, space station to therapy :-) Once attached, a connection is formed; we test out the therapist and judge them to be able to emotionally take care of themselves; they don't seem to get rattled by us and we notice that occasionally they say something that really seems insightful about us and helps us understand ourselves better. We don't know how to "get"/elicit that them-to-us good feeling experience yet but trust/see they can reliably do the them-to-us and want to watch and learn how to do that too; so that we can do the us-to-them (as well as ask for/elicit the them-to-us). Eventually it becomes pretty good, we enjoy the them-to-us when we get it and are working on the reverse. That's connection. Eventually we realize that we're working on these things AND so are they, with us! That's collaboration. Because we have these a-ha moments it makes it easier for the them-to-us learning to take place and we see/understand more of how it all works and can contribute more ourselves to the Project of Us.