What is a "just cause" in the context of Code of Canon Law, Canon 308? I want to leave a confraternity but I want to do everything according to law. I wonder what the knowledgeable people say about canon 308 and the "just cause", especially if just asking is a sufficient cause or not.

If this canon had a correspondent canon in the older laws, then I would really like to know how it was commented anciently.

Why am I asking? I'm not knowledgeable about legal jargon, but I would suspect that the intention of the law in regards to that point would be that leaving a confraternity is more of a punishment rather than a request of the confraternitee, and maybe that language implies that the possibility of an associate leaving by his own initiative wasn't even taken in account. I can't tell that if that is the case though, because I'm ignorant. My grounds for suspecting this is that it only talks about being dismissed for just cause, and not dismissing oneself.
What is a “just cause” in the context of Code of Canon Law, Canon 308?

First of all let us look at what Canon 308 actually states:


  Can. 308 No one legitimately enrolled is to be dismissed from an association except for a just cause according to the norm of law and the statutes.


A Catholic commentary say the following about this particular canon:


  Even if Canon 304, n.1 does not require that the conditions for dismissal for an association of the faithful be written into the statues, it goes without saying, since in any case they included in the conditions of admission. In effect, if people wish to be admitted into an association, they must make a commitment to espouse it’s purposes, mode of action, government, etc. However, that is not sufficient. The statues protect the rights and duties of the members, as well as of the association itself; they must specify what causes will justify a dismissal, and even the procedures that will lead to it.


Generally speaking a just cause is normally interpreted as as reasonable cause. 

We have many different types of associations in our parish. Once again our pastor has told me that dismissal from an association is done by the association against a member for any number of just reasons.

However, if a member desires to leave an association or confraternity, one is quite free to do so under no penalty of sin, since vows are not taken in order to become a member. Membership in many such said associations or confraternities, in fact are open to children. 

One is free to leave a confraternity and remain in good standing. Readmission is thus also possible, if desired at a later date.