The way I read this question is actually as two individual questions, but luckily I feel confident enough to answer both of them.
1) Why Do Rivers Flows Towards Seas?
This one is actually very easy to answer, and it all has to do with gravity. All rivers that flow towards (and eventually end up in the sea) are located above sea level, and gravity will pull it towards the sea.
However, it's important to keep in mind that not all lakes will naturally flow into the sea. Some might flow into a large lake that is located high above the sea level. The same principle applies though, and a river located above the lake might end up in the lake, but a river located below (in terms of altitude) will never be able to reach this same lake.
2) Why are rivers Never Straight?
There are few rivers that are in a straight line, but it's not impossible for it to be like this. I think you are thinking about a meandering river, which is the name we use for rivers that has a lot of curves and bends.
Let's imagine a straight river. This will have a continuous flow of water, and after a while we might see that one side is beginning to widen. This could be because of several different reasons, such as animals digging close to the edge of the river on one side, or simply from the sheer force of the water that might put more strain on one side than the other.
After one side is a bit wider than the other, the velocity of flowing water will be higher at this side, and it will cause more erosion to this side of the lake.
After several years the differences between each side can still be barely noticeable, but a lot of rivers has existed for thousands of years, or even longer than that. So after ten thousand years the very small difference might have changed into a huge curve in the river.
And that's pretty much the short version of how rivers get their bend. I hope this was clear, and let me know if you have any comments or additional questions about this. I'm not a geologist, but I have some knowledge about the matter, so I will do my best to answer.
I see two questions and they are fairly simple.
Most rivers start high up somewhere in a hilly or mountainous region are are basically travelling down hill. They are above sea level ans find their path to the sea. Water flows downhill.
Rivers are never straight and will bend etc over time to erosion. A river is like a living thing and will always fined the easiest path. When a river is formed it erodes away at the banks. Certain parts of the river flow at faster rates eroding away more. The river starts to meander and form huge curves and overtime they will break through forming a straight again . This leaves a feature behind called an ox-bow lake.
The river will change course looking for the path of less resistance eroding away what it can on its way to the sea.
Water always takes the path of least resistance.
Nature doesn't have any clear cut paths.
Bodies of water generally have an inlet, and outlet, so it's common for rivers to flow towards, and away from lager bodies of water.