As we know, citation counts are important to judge one's research activity. Is it good to cite one's previous works? Will it be viewed as an act of advertisement or self-promotion?
Although it's true that citations are always helpful, there are obviously limits. If the majority of your citations are self-citations, that's usually considered a "red flag." If your paper that came out two years ago has 10 citations, and two or three are from within your group, nobody's really going to have a problem with that. But if your paper gets cited 45 times, and 40 of them are you citing yourself, that's not so good.
Citation counts not being "in sync" with the journals they're published in are also problematic. Publishing in no-name, third-tier journal X, your paper is probably unlikely to generate many citations. It looks suspicious when such papers get many citations.
The ethical rule has to be: cite your work if it's relevant, and don't give it preferential treatment over the work of others. In short: use the same criteria for previous references to your work as you would use for citing others. No excessive citation, no self-censorship.
I think self-citation is not a bad practice in general, specially if you consider how most scientific branches work ("standing on the shoulders of giants", although you can also stand on the shoulders of normal-sized people, including yourself). However, there are two scenarios where self-citation or the lack of it can be seen as a bad practice:
Superfluous self-citation. (already discussed in previous answers). It refers to citing minutiae contained in your previous papers. For instance: "we consider the change of variable $y=x^2$ as in myself (1974,1975,1977,...,2012)"
Not using self-citation in order to inflate your results. Sometimes not citing your previous papers can produce a beneficial result. I have seen some researchers publishing a good/decent result, followed by a sequence of clones and mutant papers that do not cite the big one. This, of course, has a good effect on the child papers since it makes them look more original.
As with all referencing, the referenced works must be pertinent to what is being described in the paper referencing them. If one writes about a specific topic where much of the work has been done by the same researcher or research group then self-citing will be quite common. There is of course a fine line between that and "self-promoting" self-citing. It is impossible to try to draw the line based on number or references or percentages of the total number of references. However, it is not common that most science in a field has been made by the same person so referencing own publications where they are only vaguely related is obviously not a good way.
Having many self-citations is clearly not a sign of widely spread science, either because it is not that interesting or because the field is very isolated (or very new). Citing ones own work will definitely become obvious when looking at the citations as you have done. The normal citation index or h-index obviously does not capture this although it is possible to calculate such indecees without self-citations. But as stated above a certain quantity of self-citation is inevitable since it is likely that one publication follows on many others from the same person or group. So self-citation is acceptable to a point. It becomes less and less acceptable when the reason for the citation is pogressievly less obvious and where other papers would be equally good (or better).
Citing your previous work can be both good and bad. The biggest benefit is it might make people more aware of your work and how it fits in with a bigger topic. The risk is that people do not understand the relevance and think you are self-promoting and therefore take a negative view of you.
The worst case of this is when a reviewer tells you to cite some piece of work. If it is a big laundry list of articles all by the same author, you tend to get a little angry and think the reviewer is trying to promote that author. If it is a single article that is obviously related and the reviewer clearly states that he/she is an author, I tend to be happy to cite it.
The question you need to ask is: are you citing the previous work to promote it or to help the reader.