I'M GLAD YOU ASKED!
Yes bitterness is caused largely by hops. Why?
I'M GLAD YOU ASKED!
Hops contains resin which contains alpha and beta acids- alpha acids are the source of bitterness, beta acids do some other things.
When alpha acid is boiled, it is isomerized and rendered soluble in water. This takes a while, so bitterness is gained in beer by adding some hops at the beginning of the boil.
There are several different alpha acids- cohumulone, adhumulone, posthumulone, and a couple I can't remember. Anecdotally, the level of some alpha acids compared to some others changes the quality of the bitterness, but some studies have shown no effect, so that's up in the air.
Alpha acids are also bacteriacidal to gram positive bacteria, which promotes the success of yeast over, say, lactobacillus bacteria, which is bad. They also promote head retention and lacing.
Hops may also have some other stuff aside from the acids that add to flavor and bitterness, the jury is still awaiting research there.
The oils found in hops are responsible for the aroma quality, which varies depending on the variety of the hop.
So, what's all this mean, why am I talking about it, and what does it have to do with your question?
No idea. Moving right along.
Bitterness is good or bad depending on the style. In an IPA it's good because the beer is focused on hops, and would be boring in their absence. In a scotch ale bitterness is only there to counteract the sweetness of the malt, which is the main focus of the style.
People often times mistake other kinds of bitterness as hop bitterness. "Bitter beer face" is a good example, because the kind of beer that does this to people barely has any hop bitterness to it at all. Bitterness can also be created by cheap/poor ingredients, and bad brewing practices. These bitter off flavors are generally quite harsh, sharp, and astringent.
A lot of people 'in the know' will also be quick to quote IBU numbers. IBU is a rating of how bitter a beer is. Well, it's not really. IBU, like horsepower, is a mathematical derivation coming from several factors, and ignoring many more. All an IBU rating can tell you is its IBU rating (and even less occasionally, since there are at least 3 different equations for determining IBU). Stone can take that and suck on it.
You can compare two beers with identical IBU ratings and one can seem much more or less bitter than the other. Why? Good question. Not sure I know. So many things go into the overall flavor of beer that its hard to pin on one thing. The hop variety, freshness, boil time, amount, combined with the water chemistry, equipment, process, playing off the other ingredients, aging time, serving temperature, all the way down to an individual's taste determine the flavor you get.
So, if you like really bitter stuff, cool. Drink it. If you don't, drink other styles.