Yes, you do need to cite your own past work. Not doing so is considered to be self-plagiarism. While ethically self-plagiarism might not be as bad as plagiarizing someone’s else work, it is still not okay.
Self-plagiarism misleads the reader as it presents previously published work as new and original. Academic readers expect that all uncited material is either common knowledge or a novel idea. By not citing your previous work, you are undermining the integrity of academia.
Next, self-plagiarism is bad because publishers (including academic ones) sometimes have a copyright on your previously published work. While you are still an author you are no longer allowed to use the previous work willy-nilly. However, this also depends on what copyright agreement you have and nature of the self-plagiarism (e.g. paraphrasing old work would not violate it).
Lastly there is no reason not to cite yourself. Self-citations still improve your citation count. You should not do it unnecessarily, because overly self-citing is also frown upon, e.g., even when the previous work is not much relevant. However, when there is a legitimate reason to self-cite, there is no reason not to do it.