It is important that in public wikis, where many people work together (that often don't know each other), you always act benevolently to others, as long as no one has acted in the opposite way to you.
So, if someone deletes a sentence you wrote yourself, it might be that it was simply wrong or should not have been on that page.
In the same way, you should, of course, also exhibit benevolent behaviour yourself, and not do things like falsifying other's opinions. You should not change or delete anything unless you are sure about it.
The personal homepage should normally be considered as "property" of the particular user and should not be changed, apart from things like messages to the user.