"This Lime-Tree Bower My Prison" charts a course between isolation and imprisonment, and freedom and unity. Really, after about line 5, the speaker's imagination kicks in, and he doesn't feel so isolated anymore, but the initial feeling is important if only for the way he overcomes it. Not that he was terribly alone to begin with. He was at his own house, in a peaceful garden, and his friends were only a few miles away. But the poem argues that we can't do without social connections, even for a short time. The wrong attitude and perspective causes isolation, not actual physical barriers.
The declared feelings of isolation at the beginning of the poem are theatrical and unconvincing. Coleridge had already worked through his frustration before he even started to put down these verses.
The speaker's attempts to form an imaginative bridge to his friends are just a naïve and childish fantasy. He has trouble facing up to the darker nature of reality and human solitude.