The first factor is closeness of resemblance: the intended likeness is readily visible, as when a beautiful face is compared with the moon in a suitable context. The audience does not need to reconstruct a distant chain of associations.
The second is repeated mental presence. A comparison term may be recalled frequently through common usage or sensory experience, making it readily available whenever the subject is mentioned, even if the resemblance has several aspects.
These causes must be distinguished. Closeness concerns the relationship between the terms; repetition concerns the familiarity of the image in the mind. Both can operate together.