Some people are stronger than others in who they are. Those people should mourn, on purpose, because they will not be ruled over by their mourning. They will remain themselves even in the presence of negative feelings.
But other people are not as strong in who they are, and feelings can overwhelm them. It might be best for such people to become stronger in who they are, so that they can mourn, can see the honor in the loss of something, can faithfully register that some good is gone, or foregone.
You can feel sad in such a way that it is the sadness that is happening, and you are not really the one being sad. But in that case the sadness rules you and punishes you. So then you try not to open yourself to mourning, for fear of letting the sadness in. In the long run, you might seek some way to become a stronger person, become more personal. In a sense, to become more strong-willed, without even your own will ruling over you. But in the short term, it may be best to defend against sad feelings. Or any other feelings that possess and usurp.