Well, unless you've done several parties in your lifetime and you've gained experience, DIYing your wedding is hard. 

Speaking from over 15 years of experience, we've heard many horror stories of brides DIYing their wedding and not knowing what they're getting themselves into. Maybe your friend is designing your flowers, but most likely they'll be paid minimum or nothing at all. When friends or family aren't paid, many problems can pop up. Lack of money can cause your friend to cut corners which will take away from the potential beauty of your flowers. 

Also, who wants to work when they can enjoy your wedding day. It's hard to both work and enjoy the wedding. Lord knows how damaged hands can get throughout the process. Not to mention the puffy and frizzy hair from all the running and lifting around you'll be doing. 

Flowers are also very hard to take care of if you're not experienced and don't have the necessary equipment to care for them. You can come across wilting flowers, too few flowers, too many flowers, late arrival of ordered flowers, wrong flower colors, un-bloomed flowers, broken stems, mushy stems, falling petals, blowing petals, and so on..... 

Although we have experience, small problems still happen, but we're able to quickly fix them because we've studied and learned from experience how to deal with unexpected setbacks.