Mustard is one of those simple condiments to make, and is fun to experiment with. At it's most basic mustard is two ingredients:
There are endless variations from there.
Mustard seed comes in white, yellow, brown and black variations. I suggest buying whole and grinding them yourself using either a mortar & pestle or coffee/spice grinder. (Don't use a coffee grinder that you use for grinding coffee! You'll end up with mustard flavored coffee).
Liquids can be water, vinegar, wine, or even beer. Using vinegar/wine will help it last longer than water or beer would.
Once you've fine ground (of course there are coarse ground varieties too) your mustard simply mix it with the liquid until it is the desired consistency.
Additional ingredients can be added to taste. Honey, turmeric (gives yellow mustard its color), sugar, etc. are all possibilities.
Update: I forgot to mention that mustard needs time for the flavors to mix. This can take anywhere from a few hours to weeks. A fresher mustard tends to be hotter, but an aged mustard can often taste "better" with a slight loss of that initial heat. Refrigerated mustard will keep it's heat longer. Also note, mustard made with water should definitely be refrigerated and consumed quicker than others.
Whey appears to be an agent for lacto-fermenting mustard. This recipe for lacto-fermented mustard describes a process of combining ingredients and then letting them ferment after the recipe is put together by leaving out over a few days.
This recipe for a mustard seed raita uses yogurt as the fermentation ingredient, which means that you would have to use yogurt with active cultures.
Best Answer
As a straight answer to "why" it's the quantity of mustard oil in any given mustard type.
There are many types of mustard, but the two you may find the most confusing visually are English & American.
Though both are a fairly bright yellow in colour, that's about as far as the similarity goes.
Mustard seeds themselves come in many different 'heats' - the mildest generally being pale yellow, almost white & the strongest are dark brown.
Additionally, mixing mustard with vinegar tempers the heat & lengthens the shelf-life.
Mixing instead with pure water increases the punch, but the flavours will dissipate over a much shorter period.
American mustard is one of the mildest available. It starts with a mild seed, pale yellow/white in colour, but the colour is then boosted by using turmeric. The mixture is then diluted with vinegar.
This is your 'standard' mild American mustard, found in many places, including McDonald's. You can slather it in huge quantities on a hot dog with no ill-effects.
Almost at the other end of the scale [Chinese mustard can be even hotter] is English. Made with a mixture of yellow & brown mustard seed, then mixed with water not vinegar, this will take the roof of your nose out if used in injudicious quantities. It can quite literally make your eyes water & your nose run - though if you did it by accident, take solace in the fact that the hit is very short-lived, not like eating a blindingly hot chilli pepper.
The full impact & flavour of English mustard is quite short-lived even in the jar, once made up from powdered mustard. Even ready-made, store-bought has quite a short shelf-life for a condiment. Made at home & mixed with water it will lose its punch in just a few days, so the trick is to buy powder & mix it just 15 minutes before you eat it, for the full experience.
If you're ever uncertain visually which you are about to add - taste it first.
Personally I love English mustard on hot dogs! With ketchup too [one of the only things I will ever put ketchup on]. The balance between the sweet, vinegary ketchup & the hard hit of the mustard is really quite the feast. Bring on the junk food!
There is an in-depth article on the various types at Serious Eats - Mustard Manual: Your Guide to Mustard Varieties