This is a complicated question, and I'm sure many health professionals will have various answers, not only based on their experience, but on various studies, which are coming hard and fast as we learn more about why gluten is such an issue.
If you believe that we are holistic beings- that our brains are connected to our bodies and what we eat really does have an effect on our
brain chemistry/emotions/hormones/intelligence ( like breastfeeding can impact these, for example,) then it becomes a bit easier to accept and understand how something like gluten proteins can impact our health. There is a gut/brain connection.
In some people (and in children, until I believe the age of 12,) this barrier is open and things can cross it. This can be good and bad. For example, when strep crosses the barrier, it can cause an autoimmune response like rheumatic fever. When antibiotics cross it, they can heal more effectively, etc, etc.
When the gluten protein (and casein can be a problem as well - that is the dairy protein,) it can cause issues with cognitive abilities, or create issues like chronic fatigue/brain fog. If you have true Celiac, you are not only experiencing these symptoms, but the villi in your intestines is compromised, which prohibits your body from absorbing nutrients, which in turn means you are malnourished, have lower bone density, etc. Typically people who have Celiacs have bowel issues, and are either underweight or overweight, but not always. Sometimes they will have mild (or fluctuating) symptoms and then something like a car accident or severe stress will impact the body such that symptoms flare. (They have always been Celiac, just didn't know until the body felt great stress and could no longer cope as well.)
Why now? Why gluten? People have eaten glutinous grains for years, but you really have to dive into that to realize why things are different now. First of all, we treat our grains differently than indigenous societies. "Whole grains" are NOT the be all, end all that they are made out to be. Eating hulls of grains is pretty horrible for your bones and teeth, no matter your sensitivity to gluten or lack thereof. We also neglect to process our grains by fermenting and sprouting they way they should be. Most of the grains we eat are processed chemically with bleach, etc. not to mention pesticides. All of these toxins and inappropriate handling of grains leads to our body reacting as if grains were an invader (and rightly so!)
Another reason is that grains are heavily modified. Our diets are so full of this "super gluten" that has been created through the years (via hybridization, not the laboratory GMO we often hear about now) that our bodies are saying, "enough!" because we are getting LOADS of it each day.
If you look at the food pyramid, telling us to eat more whole grains and lots of them, and the standard American diet... cereal for breakfast, sandwich bread for lunch, pasta and more bread for dinner.... not to mention all of the hidden gluten (gum/candy, body products, lipstick, etc.,) it's easy to see why we are on overload!
I went gluten-free 2 years ago after several doctors suggested it for my daughter (due to her autoimmune issues.) Our entire family went gluten-free so that we could all eat the same things and no one would feel left out of treats, etc. We immediately felt changes that were completely unexpected. My husband's chronic foot pain went away and his sleep apnea completely disappeared. We lost weight (about 30 lbs each the first year) and have kept it off! I was able to take a deep, full breath for the first time EVER (I didn't even know I had breathing problems!!) and my stress decreased significantly. BOTH of my daughter's attention spans improved, as well as their sleep and frequency with which they catch colds/severity, etc. (still not perfect, but drastic difference.) The list goes on and on! I also have NO period cramps after years of suffering, no PMS, and my acne cleared up within about a month.
All of this to say I didn't hop on the bandwagon to lose weight or anything like that. We aren't diagnosed with Celiac (though I wouldn't be surprised if I and one of my daughters had it, due to our symptoms.) It's not a fad for us or something we take lightly. We don't "cheat" and have been gluten-free for 2 years and plan to NEVER eat gluten for the rest of our lives.
I'm sure for some people it is trendy, but it's also very serious for some. There is NO harm in going gluten-free that I have been able to find (I did pretty significant research before we started the diet.)
Hope this helps. You can google most of these concepts and find relevant material from reliable sources. I didn't include any because I was sure someone would question my sources, so I will leave it up to everyone to do their own research. The one thing I will mention about the soaking/sprouting grains and eating whole grains is the Weston A Price site has a lot about this, or the book Nourishing Traditions.