One way to understand how the different signs interact with one another is to take a figurative look at their respective elements.  We have Fire Signs, i.e. Aries, Leo and Sagittarius; Earth Signs, i.e. Taurus, Virgo and Capricorn; Air Signs, i.e. Gemini, Libra and Aquarius; and Water Signs, i.e. Cancer, Scorpio and Pisces.  So what happens in Nature when any given two combine?  This exercise can be fun and enlightening at the same time.

Naturally, there is no combination that represents a perfect match all the time, though some go together better than others.  For example, as a rule, Fire mixes better with Air than it does with Earth or Water.  When you think of a fire, this makes perfect sense.  It needs air to burn; if you want to put one out, you douse it with water or dirt.  Likewise, a Fire Sign can be fed by Air, or put out by a Water or Earth Sign.  However, too much Air is not necessarily good, either.  It may be required to burn, but being given too much can result in a raging inferno.  Earth and Water tend to go well with one another as well, when you think in terms of what is needed for a plant to grow.  In general, Earth and Water Signs get along well.  So that's the idea, which is explored a bit more in the links below. 

Like most analogies, these break down at a certain level, but they can give you something to think about regarding what the relationship dynamics will be when these basic personality types get together.  There are additional things to consider that will affect compatibility between any two signs, but looking at the elements involved gives you something with which to start.  These should give you something to think about in a very fundamental way regarding with whom you get along and perhaps why you're having problems with someone in your life.  These interactions apply to all relationships, not only of the significant other variety; they apply as well to close family members, friends, coworkers, and even that person in front of you in line at your favorite restaurant, Wal*Mart or the grocery store.

If you're a visual type, thinking of a relationship as a living entity being fed by the two individuals involved can help you to understand what works and what doesn't as well as what might be wrong when problems arise.  If you think of the major characteristics of the two individuals it will help you understand who's in control and what behaviors need to be modified to attain the harmony you desire.  In some cases it might not be worth the hassle, but at least you'll understand what the basic problem is.

The Modalities of Cardinal, Fixed and Mutable (also known as the Quadruplicities) come into play as well, but it's surprising how much the astrological Element alone can predict.  Find the combination you're interested in and then click the applicable link in the headings below.  It shouldn't take you very long to see what I mean.