So here comes the boring science stuff, or fun stuff if you like science. I won't get into "why we kiss", because that could take a long time and it's really not that interesting. What is interesting though is the "high" we experience when we are having a good long kiss with someone. Yep, kissing is like taking a drug. Our lips are rich with nerves and these nerves send information to our brain when we kiss. The sensation of kissing sends a message to the pleasure center in our brain, which then signals a release of feel good chemicals like dopamine and endorphins. Hence we feel euphoria in a really good kiss. Over a period of time, kissing actually triggers our body to produce less cortisol, which is the body's stress hormone. So kissing boosts feel good chemicals and decreases stress; two things that helps us feel less depressed/anxious/angry. You can see why this can have a positive long term effect on our physical and mental health.
In a romantic relationship, kissing is a very intimate act. Just think about it, you need to feel trust and comfortable enough with someone to bring them that close to your face. When you are that close, you can hear, smell and see every little thing you wouldn't have noticed three feet away. And of course, you also put yourself in that vulnerable spot of being smelt and judged from a distance of three inches. Then finally, when you do kiss, you taste that person. What is more intimate than tasting someone? Although I don't have information about whether or not a relationship/marriage has ended over bad kissing or lack of kissing, I can say that kissing is good for a relationship. Not necessarily because kissing is magical, but because of what it represents. A kiss is an expression of love and affection. Its presence expresses the continued love and affection we have for our partner, and I can say that the continued expression of love and affection has a tremendous effect on a long lasting relationship.
So ultimately, if there is a decrease in kissing through the course of your relationship, then find other ways to express love and affection for your partner (ie. leave the toilet seat up, text a love message, encourage his/her dreams). Find what suits you. Just keep in mind that kissing is a very effective way of showing love and affection as expressed above. However, depending on your culture, age, or preference, you may or may not enjoy kissing at all. But clearly, if you do enjoy it like millions of people do, then you owe it to your partner, your physical and mental health to maintain it in your relationship.