In real life--ie, not your job--you often explain why you're doing things as part of informing people that you are doing things. For example, "I can't make that weekend to go out because we're having a house guest over from out of town."
Sometimes in work life, its important to explain a choice or decision similarly: "We cannot go with the blue color scheme because it looks too much like our competitor; please see the attached image copies."
However, its important that in professional and business communication (and to a certain degree in private communication), that you don't always include the reasons why for things...or even how. Often when and where will get you a lot further without starting a discussion about the other things.
For example, telling your boss that you're suddenly free on Monday because the client cancelled the all day meeting seems innocuous enough...except your boss (or anyone else) might wonder WHY the meeting was cancelled. You end up having a long discussion about the fact that you didn't do anything wrong, that the client had a conflict, and may even end up explaining the death of the main developer's cat to multiple people during the day. Instead, "Monday works fine" would have been a much better "when can we meet" than "Monday, because the client cancelled." If there are questions (such as people remembering you had a meeting and now you don't), you can take them off the main email (where lots of other people are) and answer individually.
Generally, when trying to decide how much to include in communications is dependent on the number of recipients of that information, their investment in the information itself (perceived or actual) and whether producing that information will help or hurt progress on the existing items or any future ones.
For example, an email to a large group should be succinct and not trigger additional questions that people will (because they naturally do) hit reply all and ask. Try to leave justifications and explanations out of emails to large recipient lists unless its a summary of what you've all agreed to or some subset has agreed to and agreed to pass on to the rest in this form. So, "free lunch today at 1 pm in the break room" to the entire team is way better than "free lunch today at 1pm in the break room because the sales meeting cancelled." There will be people who want to know why it cancelled. They may email or email you or email sales. Some might suddenly worry the company has lost a client. Keep it short, sweet, and if there are additional questions, take off reply to all and reply individually.
Investment in information is often hard to gauge, but also a strong indicator of reaction to reasons, explanations etc. in your communications. A complete lack of investment--"Why do I even care?"--may be the response to such information, which is bad for morale and can hurt trying to get them invested in the future. For example, "Hey Bob, we're all going to this symposium on Java, wanna come?" might be more effective than "Hey Bob, we're all going to this symposium on Java because the company is transitioning from C++ to Java and you need to know this stuff in six months to a year" for some people. Specifically people resistant to change, or ready to road block it entirely. It doesn't mean you don't tell Bob about the change, but you don't tell Bob about the change at the same time your trying to invest him in that change. That typically spells disaster; either Bob will go out of fear and not be his best, Bob will go and be passive aggressive about, or Bob will not go in hopes to affect the outcome of the transition (ie: make it not happen/make it not real to him). Bob might be the type that would go to prep for the transition, and if he is, then he's already invested (at least as much as the inviter knows), and the full reason for inviting could be included.
Finally, does the information or lack thereof hurt or help the point of the communication? Talking to folks at a higher level than yourself, it helps to include that someone at their peer level or higher okayed the decision and requested you pass it on/they do the specified task. Talking to folks at a higher level having a political pickle with the person who requested you pass on the information or they do a specified task might encourage you to leave the name of the person who made the request out...at least initially. This is because it could hurt their compliance with the information or the task associated.
In general, you want to pass along enough information so people can invest in what you're passing on and make informed choices about understanding as well as complying with any task requests. You don't want to be guilty of withholding information, but at the same time, you don't want to include information that could be confusing, start a fight, or otherwise cause problems for you, the information, existing tasks, or future ones.