First of all, don't panic! Gauging the potential for a fulfilling romantic relationship from your signs alone won't work. Even if he or she is the 'right' sign for you, there could still be problems.
If you really want to know more about the potential for a happy love life with that special someone, you need a more detailed chart comparison, which will allow you to see things in perspective.
This is a branch of astrology known as synastry, which enables the astrologer to look deeply into the hidden motives and agendas of both people involved. They take a look at the good points of the relationship and compare them to the more difficult areas and discuss the pros and cons of getting involved.
They also take a look at the transits under which the relationship began and see if these are harmonious. For instance, one of the more difficult transits for starting a relationship is Neptune conjunct the Descendant - the point opposite the Ascendant. The relationship may begin for all the wrong reasons - muddled motives. The initial feeling that you have met the perfect person may rapidly be replaced by the knowledge that he or she is not anywhere as amazing as you thought.
The same is true for relationships begun under powerful Pluto transits which may descend into power struggles or become obsessive. So, an incompatible Sun is probably the least of your problems and may even turn out to be of real benefit.
Very often people are attracted to those who have qualities that they lack. This is very apparent when comparing the charts of two people, especially where the balance of the elements is concerned. Someone who has an abundance of fire and air may attract a lover who has a lot of earth and water - they both compensate for each other's weaknesses.
A couple may have compatible Sun Signs, but if they have a number of challenging inter-aspects then the relationship is going to be hard work. On the hand, some couples have synastry charts that are quite difficult and yet they get on like bread and butter and stay together for many years.
Another way of finding out how well a couple will get along is to draw up what is known as a composite chart. This is a chart of the relationship. It is created by calculating the midpoints of the various planets and points of the two charts, thus creating a third chart. This doesn't so much highlight the individual viewpoints of each partner, but shows the quality of their interaction. From this it is possible to gauge whether the relationship is likely to be long-lasting, intimate, businesslike, or platonic.
The more exact the aspects, the more powerfully that particular quality will show up in the partnership. For instance, composite Sun conjunct composite Venus indicates the couple has a genuine fondness and real love for each other. Composite Venus in opposition to composite Pluto suggests power struggles as a theme running through the relationship. Composite Mercury in aspect to composite Mars suggests intense discussions and a penchant for arguments.
Overall, it is a question of looking at the inter-aspects and the composite chart and weighing the various influences to get an overall idea of the level of compatibility. It also helps to listen to your heart!