Windshields are not super complicated things. What makes windshields crack and break is usually one of three things: weakening due to age, direct force, or weather-related trauma. In some cases, a crack can be repaired, but in most cases, an actual crack (rather than a chip) means a windshield replacement.
Windshields crack due to time and age, usually precipitated by an event. For example, over time, you will receive thousands of tiny hits on your windshield: small rocks, grit blown off the road or kicked up by passing vehicles, sand and salt from winter road icing, and even windstorms. All of these tiny hits gradually pit your windshield so that a bump in the road can precipitate a crack that spans four inches or your entire car width.
Another cause of windshields cracking is direct force. That extra-large rock that gets thrown up by the big vehicle in front of you. That baseball at a fall game. The icicle off your car port. Someone throwing cans off an overpass. Any numbers of direct hits can cause a crack in your glass.
Finally, weather. If you have a small chip, the changing weather can turn it into a large, inconvenient crack. The cold, cold nights followed by direct sunlight that heats up your car quickly can play havoc with chips that you didn’t even know existed until they turn into cracks.
If the crack from any of these causes is small enough (<4 inches), you might be able to get them repaired. If they are not right along the edge of the glass, and if your windshield is not terribly old and pitted you might be able to get the crack repaired.
In many cases, however, a crack—unlike a chip—really means the windshield needs to be replaced so it continues its job of keeping car occupants safe.