Katherine Anne Porter's "Theft" is an interesting short story about adult life, especially all of the relationship and financial hardships that accompany adult life. While it's true that the unnamed heroine is lonely, we actually learn through the text that she is not lonely because she has been rejected; it is she who has caused her own loneliness, who has stolen from herself all that's valuable to her.
There are definitely quite a few important passages that help portray the heroine's loneliness, especially towards the end of the short story. The first can actually be found in the letter she reads from some unnamed person. However, from the contents, we know the letter is from an ex-lover. We might even presume it is from who she refers to as Eddie, since it is evident Eddie is important to her due to the fact that she spends a great deal of time comparing Camilo's ability to look good in a hat, especially a warn out hat, with Eddie's ability to look good in one. We are not told much about the letter, but we are told it states the following partial sentences:
"thinking about you more than I mean to ... yes, I even talk about you ... why were you so anxious to destroy ... even if I could see you now I would not ... not worth all this abominable ... the end ...."
From these, we know it is a letter concerning her breakup with this unnamed man, possibly Eddie. More specifically, we know that it was actually she who broke up with him based off of the partial sentence, "why where you so anxious to destroy ...." From this partial sentence, we can guess the writer of the letter had written something like "destroy our relationship" or "destroy our love." Hence, we know it is this unnamed man who blames the heroine for the destruction of the relationship. What's more, we know that he is absolutely torn up about it, and as a matter of pride, he wouldn't take her back now even if she wanted to repair the relationship. Hence, this is an excellent passage that describes her loneliness, but it also shows that her loneliness is her own fault, not due to rejection.
Another important passage describing her loneliness can be seen just after she confronts the janitress about the stolen purse. In a very long and important passage, she reflects on all the valuable things she feels she has been robbed of. More importantly, she ends by stating she also feels she had been robbed of love, as we see in her passage:
... the long patient suffering of dying friendships and the dark inexplicable death of love--all that she had had, and all that she had missed, were lost together, and were twice lost in this landslide of remembered losses.
Clearly, the references to dead friendships and the "death of love" are descriptions of her loneliness. More importantly, in connection to the letter, we can understand the phrase "she had had" to be referring to the relationship she had had with the unnamed letter writer. We can also understand what "she had missed" to refer to, as the letter writer phrased it, how she had "destroyed" their relationship because for some reason she had been too blind to see its value. Hence, while the passage portrays her loneliness, it also places blame for that loneliness on herself.
The final important passage that not only describes her loneliness but places blame on herself for that loneliness is her final thought to herself: "I was right not to be afraid of any thief but myself, who will end by leaving me nothing." In this passage, she is calling herself a thief, a thief who has robbed herself of all things valuable, like her relationship with the unnamed letter writer, which, again, clearly shows her loneliness but also the fact that she herself is the cause of her loneliness for having destroyed or rejected the relationship.