Writing a paper as a single author in 1st person singular
I am not a native English speaker, but I don't think this is important here. The appropriate thing is not what an English professor or a poet suggests, but what is standard in the particular field. And from my experience, "we" is the standard in theoretical computer science. As noted by usul in his comment, you can interpret the "we" as the author including the reader. I can think of several reasons why "we" is better than "I":


It sounds more natural and less egocentric.
A paper in theoretical computer science is not about your personal opinion, but you state and prove general facts. Replacing "we" with "I" in terms such as "we thus have" is therefore weird since what follows is generally true, not only for you.
Submissions are often anonymous. You don't want to reveal that the paper has been written by a single author in the submission (and you don't want to change "we" to "I" in the final version).


Edit: An exception are parts of the paper that are subjective, e.g., the acknowledgement section: I would like to thank András Salamon and Boaz Barak for pointing this out.