If the speaker of Sonnet 29 had a theme song that he performed every Friday night at his favorite karaoke bar, it would probably be "All By Myself." (You know, that pathetic song Bridget Jones sings while she's all alone in her apartment wearing a pair of ratty old pajamas?) For the first 8 lines of the sonnet, our speaker insists that he's got zero friends and that God has been completely ignoring him. There's evidence that our speaker is physically isolated as well—the sonnet makes it clear that he's been separated from someone who loves him. In the end, the speaker finds comfort in his memory of the person's love. In the end, we get the sense that loved ones can be with us in spirit, if not physically present in our lives.
The speaker is physically and emotionally isolated because he's a social outcast (not the good kind of outcast).
The speaker of the poem claims to be a social outcast, but it seems like he's just being a drama queen (or drama king?) and has isolated himself from the rest of the world.