When considering the matter of undoing self-deletions there are in mind at least three interests to balance.
- the author
- the site as a whole
- other individual users
The author: As for editing, I think trying to respect an author's intentions is an important goal. I observed on various occasions (mainly on other sites) that authors have quite strong feelings when it comes to their self-deleted content.
It seems by some it is perceived as a considerable loss of control and possibly even breach of trust not to be able to delete something they want to delete, and even more to have something reappear they thought deleted.
The site as a whole: while (most) posts are assigned to a specific user the site is also a collectively built resource. In contributing to it one makes a donation to this general resource and as for donations in general it is then not always possible to ask it back at will. For example, arbitrarily deleting some answer that is relevant to some thread might degrade the quality of that thread even below the level it would be at had the answer never existed (as the earlier presence of the answer might have made others not give theirs, for example).
Other individual users: deleting a question, does delete its answers, too. The same goes for posts and their respective comments. Thus deleting is not only a personal decision but might directly effect others.
The technical context
Almost completely deletion of questions with answers is prevented by the software (a single upvote on one answer, or two or more answers will block it). Comments do not prevent self-deletions; however, one should in general not rely too much on comments for substantive contributions.
(Moreover, accepted answers cannot be prevented either.) These technical measures protect quite well other individual users interests, and the sites interests to some though to a lesser extent.
There are also some technical measures in place against massive deletions.
I will not undo self-deletions that seem reasonable (only those out of vandalism or spite would be reversed). In case I have doubts about the motives I intend to ask. (Just in case, the mere fact that I might ask does not mean I consider the action as unreasonable; sometimes a self-deletion could also be an outgrowth of legitimate unhappiness about others actions related to the post for example that I might wish to know about.)
If you come across a self-deleted post and want to inquirer related to it, I recommend to use a mod-flag. In general I think it is good to discuss things in the open, however especially for deleted content it can be better not to draw attention to it via a meta-post. Similarly, I would like to ask users with undelete-privilege to be very constrained when it comes to undoing self-deletions; this is a rare case were I would prefer you also flag for moderator attention instead of using the tools you have (there could be some context that is not apparent).
If you intend to self-delete something, please keep in mind that there are also other options, namely you can ask for the post to be disassociated from your profile (this is bit complicated though but in isolated cases it is an option). Or, if for example you are happy with your post but frustrated with recurring comments to lock comments can also be an option (flag for moderator attention); this should be reserved for cases were there is demonstrably a problem with comments not as a preemptive measure.