What Makes Something Sinful?
The simple answer is that we sin whenever we disobey one or more of God's commandments.Unfortunately, like most simple answers, that doesn't really explain anything. If you're not already familiar with God's commandments, then it's very difficult to understand why some things are sins and not others. This is not helped by the fact that there are people who label anything they don't personally like as sinful.
But there is a way to solve this problem - check your Bible.
In Mark 12:28-34, a teacher asked Jesus which of God's commandments was the most important. Jesus responded that the most important commandment was to "love God with all your heart, mind, and soul". He then continued by adding that the second most important commandment was to "love your neighbor as yourself".
Since all of God's commandments are summarized by one or both of these statements, we can infer that every sin ultimately involves one of two things: either it's a sign that we love something more than God, or it'll hurt someone. Keep in mind that this doesn't just mean physical harm - many things are sinful because they hurt people emotionally.
We can see how this works in practice by looking at the Ten Commandments. Although these are not the only commandments God has given us, it's easy to see how we'd hurt someone by stealing, lying, cheating, or commiting murder. We can also see that God doesn't want us to work ourselves to death - the Sabbath is, after all, an official day of rest.
So, if you're ever wondering why something is a sin, look around and see how it would hurt someone or your relationship with them. If something really is sinful, then you shouldn't have much trouble finding the answer.