Not necessarily either.
Real astrologers do NOT compare signs for compatibility.
Nor do they work off of Sun-signs alone for ANY astrology (the sign you find using just the month-day of birth is the Sun sign).
The Sun sign is only a very small part of the total picture, and how any two people interact, astrologically-speaking, is shown by the "synastry aspects" between ANY of your 10 planets (not just the Sun) and any of the OTHER person's 10 planets.
Always there are some synastry aspects that show attraction and harmony, but also some that show discord.
What makes it or breaks it is now how much love there is, but how emotionally mature both parties are. Whether or not the discord upsets and sinks the relationship.
Astrology is only an influence and it shows only how the person will be IF they do nothing to grow up.
So you cannot tell by looking at astrology how much THAT particular individual has outgrown their natal chart. You have to talk with them.
And astrology cannot tell you if two people can make it work ... that depends on their maturity levels.
So why would anyone bother? Well, once the relationship has lasted long enough that the honeymoon phase has died out and the problems have started, the synastry evaluation can help you understand WHY there is conflict. What needs of yours aren't being met and so you react badly ,and what needs of their aren't being met and so they react badly.
So you have some clues as to the cause of the discord, and this may or may not help the couple figure out how to work around it and to build a solid lasting relationship.
NEVER count someone out just because of their Sun sign. Especially since that is NOT what makes it work.
My in-laws, married over 70 years before death and married very happily ... had Sun signs that were supposedly incompatible, according to the scam of "Sun sign astrology". He was an Aries, she was a Capricorn.
But when you sat down and look at the synastry aspects between all their planets, you saw that there was discord, but you also saw a strong lasting tie between the two charts .. not involving the signs at all, actually.
That and the fact that neither of them were too badly damaged, emotionally, and that they stuck if out and figured out how to get along and accept the imperfections ...
They had a beautiful marriage.