Answer: The Torah forbids a Jew to take revenge for some hurt he has suffered. Although the classic example of revenge cited in the Talmud deals with monetary matters ("I refuse to lend you the tool you request because you refused to lend me your tool when I requested it of you"), the mainstream view of halachic authorities is that the prohibition against revenge applies to all matters of human relations. Even if you go to that Bar Mitzvah but cant resist telling the host that you came despite his not coming to your simcha, you are guilty of violating the Torah prohibition against harboring a hatred for the person who offended you.

All of this applies, however, to someone who ordinarily attends any simcha to which he is invited. If, however, you are a very busy person who finds it difficult to even attend every simcha of friends and relatives, but feels an obligation of gratitude to attend the celebrations of those who participated in the ones you hosted, the decision to absent yourself from the simcha of one who did not make that gesture cannot be considered a forbidden act of vengeance.

Although this is the halachic norm, you should be very careful to search your soul to ascertain that you are not acting out of the slightest motive of vengeance and even then try your utmost to go beyond the letter of the law by attending your acquaintance's simcha so that there should not remain any trace of retaliation.