I am an Orthodox, Ashkenazi Jew. However, I find it hard to understand why, in certain cases, if an Ashkenazi is forbidden to act one way, it would be permitted for a Sefardi? For example, a Sefardi who consumes kitniyot on Pesach is not committing a wrong due to the fact that his minhag allows for it. Yet, an Ashkenazi who eats the very same item has just done a transgression, due to the fact that his custom forbids its consumption on Pesach. How can the same act be permitted for one segment of Jews and forbidden for another?

Your question is a very tall order, but I will try to give you at least a partial answer.

It is very unfortunate that we are currently in golus (exile). In the ideal situation, any questions in Jewish law would be posed to the Sanhedrin (the high court), and their ruling would be binding on all. Since then, there have been disagreements amongst the sages as to how/what to do. There were some communities that followed their local sage, and others that followed theirs. Up to the time of the completion of the Talmud, the sages did get together, and, at those times, rulings were made on most of these cases with the majority opinion being binding on all. After that time, the exile has made it that each community follows its individual rulings, and, with few exceptions, those rulings are binding on that community; but not beyond it—even where the majority is one way or the other. When a person moved (permanently) from one community to another, he would take on the minhagim and rulings of his new community and drop all previous customs. The exile has gotten worse, and we no longer have a such a strong sense of community. We live with many communities jumbled together, each keeping its own minhagim and rulings. However, in some senses, we have maintained our community identities. All Ashkenazi Jews are considered to be one community for those rulings and customs which all (or most) Ashkenazi Jews have adopted; and the same is true of Sefardi Jews. This holds true when there is an independent community (i.e. a synagogue) for each group. (It may require more than just a synagogue for the implementation of certain community-based customs, such as certain minhagim pertaining to kosher laws. Speak to a competent authority of Jewish law for more details.)

With regard to your example of kitniyot on Passover, the only case in which you could change your custom is if you were to move to a country where there is no Ashkenazi community. In fact, in this case, you would require specific circumstances in order to maintain the stricter custom of your father. However, the necessary conditions required for this are beyond the scope of this format.

I hope this is helpful,

Rabbi Y. Benyowitz