Why don't wind blow in a straight line?

How many years do you have for the answer? I can give you the barest concepts, but that is a very deep subject. I'll start with the largest scale issues and then develop some of the concepts that control mid-latitude synoptic scale flow. On a large scale, the wind in the tropics comes from the east, while in the mid latitudes it comes from the west. This is a manifestation of angular momentum conservation. At the equator, the earth is moving at about 1670 km/hr from west to east, while the speed drops with the cosine of the latitude, so at 45 degrees north (or south) it is moving at 0.707*1670 = 1180 km/hr. The air largely moves along with the ground speed, but in the mid latitudes there is air mixing from the tropics, which causes it to move a little faster than the ground, so it runs ahead, which means it seems to come from the west. The air in the tropics is moving a little slower that the ground, so it comes from the east. In the mid latitudes, the average day would have the air coming from due west. For complex reasons the air tends to form wavy loops, so that at some points it blows from the northwest and sometimes it blows from the southwest. Mid latitude easterlies aren't impossible, however. Usually they come from a cyclonic (counterclockwise in the northern hemisphere, clockwise in the southern) passing on your equator-ward side. Put your right hand on a weather map (left in the Southern Hemisphere), with the pinky side of your hand on the map and your thumb pointing up. Your fingers show the way the wind blows around a low. Even though the large scale flow is from west-to-east, locally the wind can be from the east.