While you most certainly can write your own will, I believe that there are too many potential problems that can arise.
A DIY attitude can get you a lot of places, after all, its part and parcel to the entrepreneurial spirit. Of course, anyone who's jury-rigged a project also knows that a DIY attitude doesn't always translate into a successful end result. In the end, you either learn from your mistakes for the next project or else hire a professional to do it right the first time. What about your Last Will and Testament? What are the dangers of a "DIY Last Will"?
Forbes recently ran an article on the possible pitfalls of a DIY will and the reasons to consider a professional. It can't be denied that it is easier now than ever before to prepare your own Last Will and other estate planning documents. Welcome to the computer age. However, unlike other projects where you can learn a lesson for next time or, conversely, learn to hire the pro if things don't turn out right, you won't have a chance to "fix" your Last Will if it proves unsatisfactory.
By the time your Last Will becomes truly important you're no longer around to tweak, to re-align, or to call in the experts. For one thing, the assumptions you might have today (e.g., that a younger relative or child will outlive you, that an asset you own today will still be around tomorrow, and that all the details will fall into neatly into place, etc.) could end up falling short, as would your plans. Likewise, you might be fully confident regarding the meaning and intent of your "language," but others may interpret a different meaning or even argue that you didn't specify well enough (i.e., "legalese" can be a difficult thing to DIY).
In the end, having a professional around to defend your choices and your intent in the face of unforeseen changes can help ensure that even the improvised bits of your estate plan will work out in the end.
Indeed, there are a lot of reasons to consider consulting a professional when it comes to your Last Will (or for any legal matter), as the Forbes article explores in greater depth.
Reference: Forbes (November 18, 2011) "5 Reasons to Get Professional Help Writing Your Will"