Hindu wedding is not just a tie up between two people who fall in love. It is a marriage based on a strategical calculation of gunas, these gunas indicated about the compatibility of two people in various aspects. That is why marriage as per Hindu tradition is advised only after matching the horoscopes of the bride and groom so that they can live a compatible and happy life.
Now coming back to your question.
If only less than 18 gunas match between the bride and the groom, the marriage cannot be successful and hence pairing such individuals is never advised as per the Vedic astrology. If 18 to 24 aspects match, the marriage can be approved. However, the results of compatibility will be average. Matching of 25 to 32 aspects is considered a very good match with the prospects of the couple assuring a very happy marriage. If 33 and more aspects match, then such pairing is considered the best on the earth and can give way to an excellent marital union for the individuals coming together.
Another important aspect while checking marriage compatibility is ‘Manglik Match’, which determines well-being and the success of the marriage. Hence even if a couple has 36 out of 36 score , one cannot guarantee that they will have the perfect marriage. In simpler words, Gun Milan is a thorough speculation to measure success of marriage in future otherwise issues such as difference of opinion and ego problems can make the marriage fail eventually.
The system of kundali matching is based on very ancient tradition and practice and is the gift of Vedic astrology to the humans. In fact, Kundli matching is a science not a mere superstition. On a subtle analysis, the findings of the Ashta Koota matching have been proved to be highly effective in paving way for happy marriages and hence taking inputs from the Kundali matching is a sure way for the success of marriages.
In astrology, a 36-point compatibility matching is not the only method for predicting about a successful marriage. Neither it is a full proof method because it is only based on the placement of moon but there are many other planets in the chart who play an equally significant role in the married life.
It is only the emotional compatibility that shown by placement of moon but it cannot give the true picture. Therefore even if a couple have 36 out of 36 aspects matched, one cannot guarantee that they will lead the perfect marriage life. I have charts of some couples who have scores of less than 13 gunas, and yet they are living a happeir life.
The 1st, 2nd, 4th, 7th, 8th and 12th house play an important role in married life. If malefics placed in these houses it will give bring bad luck and misfortune especially in the areas of marriage. Hence if placement of malefics exist in a chart of one partner then the chart of the other partner should be looked accordingly. This method is known as 'dosha samyam' in which we compare doshas present in the two horoscopes.
Also all those planets that involved in good and bad combination are also judged from marriage's perspective. Apart from that, the placement of every other planet in the two horoscopes should be judged especially those in bad combinations to ensure that 'no partner to be suffer for a very long period of time'. Other important consideration such as the relationship between the two lagna lords and the character of both the partners should also be taken into account to check the tendency of loyality between the partners.
I'm in this field of astrology from past 17 years and give marriage consultation to married and unmarried couples. From my experience I can say that, if both the partners have some degree of faith in the institution of marriage then it is easier for them to go through tough times because both of them are willing to talk about it and make it work. While I have seen many couples who are impatient and ambitious, to wait for good times to come.
That is why it is extremely important to look at the entire horoscope to interpret the condition of marriage and not just the matching of 36-point aspects.