Why? Technically, the reason is that the StudioPro-widget for selecting an entity simply shows all entities for which there is only one associated object related to the top entity, and then for each of those entities it again shows all entities for which there is only one associated object, and so on. Since a 1-1 relation goes both ways, it results in this infinite loop.
Why, logically? Can’t think of any use-case. You will be going back and forth over the exact same association, so there is nothing to gain by selecting the top object over a path that is first travelling over a 1-1 associated object.
It is just a strange unuseful result of the StudioPro IDE which most of us wonder about the first couple of times, and then just disregard it.