Kissing is Not Always a Sin
First off, there are different degrees of kissing. Jesus kissed his disciples. We kiss our family members. Even in Europe kissing is a common form of greeting among friends. So kissing is not always a sin. However, it's not likely someone is going to ask if kissing as a greeting is a sin. The real question, then, is whether or not romantic kissing is a sin.
When Kissing Can Become Sinful
So, now we are down to the real question of if kissing is a sin, and the answer really comes down to what is in your heart at the time. The Bible clearly tells us that lust is a sin. So, is lust in your heart when you're kissing? Is the kiss making you want to do more with that person and leading you to temptation? If the answer to these questions is, "yes," then kissing can become sinful. It doesn't mean all kisses with someone you love or are dating are sinful. It just means to be careful about what is on your heart and to make sure you have enough self-control if you allow yourself to be in a kissing situation.
So, Should I Kiss or Not?
The answer to this question is up to you. Some people choose not to kiss until marriage, because they see kissing as leading to sin or they believe romantic kissing is a sin. Others feel that, as long as they can overcome the temptations and control their thoughts that kissing is okay. The key is to do what is right for you. This means spending time praying about it and thinking it through. Sure, "everyone may be doing it," but that doesn't mean it's right for you. Do what God lays on your heart.