"Joyous practise" must be the rule for all who will make aikido a lifetime habit but we need to develop personal control. There is nothing inherently wrong with a smile on the mat or even a laugh but we should also develop the ability to feel joy without showing it on our faces and in our voices - that is simply good practise (ask any poker player) because there may be times when it is inappropriate to laugh. For instance, a laugh or a smile might be misinterpreted by your partner or your sensei because you can never fully know the other person's mindset and an innocent giggle, just like an innocent remark, can sometimes cause a problem and if that problem comes as a surprise to you then in some instances at least you need more self-control.
The simple fact is that we provide information to others by the words we choose to use, the tone of those words and our body language and as martial artists we should have enough self-control to ensure that everyone around us is getting the right message, whatever it is.
yours in aiki