Astrology can be a very complex science. There are a great deal of aspects to consider when determining compatibility between two signs. One of the best, easiest, and often overlooked way to see if two signs are compatible is by simply understanding the elements.

There are four elements, and each one is attributed to three specific signs. They are as follows:
Fire: Aries, Leo, Sagittarius
Air: Gemini, Libra, Aquarius
Earth: Taurus, Virgo, Capricorn
Water: Cancer, Scorpio, Pisces

Now each of the elements have certain characteristics, and I'll give you a brief summary of what those are.
Fire: Positive, Assertive, Determined, Active
Air: Social, Intelligent, Doers, Communicators
Earth: Stable, Enduring, Structured, Introverted
Water: Passive, Receptive, Reactive, Intuitive

It's possible that you are beginning to understand which elements are the most likely to blend well together.