The speaker is describing how love arrived slowly and gently, almost against their own instincts. The image of a "frightened puppy" coaxed out of its shell is a slightly odd, mixed picture, since puppies don't have shells, but it gets the point across: this is someone who was timid and guarded, and who had to be drawn out with patience. The word "learnt" matters too. Love here isn't a lightning strike but something the speaker has gradually come to understand and accept.
Once that happened, the feeling settled into something permanent. The speaker has "built my home around" the other person, and the smile they mention is described as easy, which suggests it's part of what makes them feel safe. The title, "Harbor," fits this sense of shelter. The mood is tender, quiet, and grateful. The poem ends by saying the heart "has found home," leaving us with a feeling of relief and contentment.