。
That's because you don't want to be alone.
Literal
That [topic-は] you [subject-が] one-person [target-に] become-want-not [reason-から] [copula-です].
The structure ~から+です ('it's because X') is the standard pattern for offering an explanation. Inside, 一人になる ('to become alone') is the natural Japanese way to say 'be alone' — emphasizing the change-of-state with なる rather than a static 'is.' ~たくない is the negative want-form. Note that が marks the subject inside the embedded clause, while は marks the topic of the main sentence.