Choosing a fishing line is just as important as your lure selection. Most people select a a line that is too heavy for their needs. People will say why not go heavier? I say "why not just wear your winter coat all the time?"
The lightest line you should use is 4 lb. I do not recommend using 2 lb line in any situation. Its just too light. It stretches, curls and breaks too easily.
When Trout fishing the decision is really between 4 lb and 6 lb. The first factor is the weight of the lures you are using. If you are throwing 1/4 oz spinners you will have no problem with 6lb line. If you are using lighter lures like 1/16 you should really be using 4lb line. I personally use 4 lb line unless I am using 1/2 oz or heavier. Another factor is budget. I replace my line every time I fish. 4 lb line will stretch and when it stretches it curls. Think how your Mom curled ribbon on the Christmas packages. If you find that your line is constantly catching over the last loop on your rod, then its stretched out. If you are using 4lb line you need to cut 10 feet of line off every time you strain the line. That means after a large fish and after a snag. If the line curls you are going to waste a lot of time with real problems and the line catching on the last loop.
So why do I use 4lb line? It's because I can cast a lure into a bucket 50 feet away. Yes I practice. The lighter line has less resistance when you cast. Its more accurate and casts further.
If you have the budget use the 4 lb line.
If you aren't looking to spend that much on fishing line, use 6lb line. Just use 3/16 oz and heavier. 1/8 and less is going to be too hard to cast.
If you are loosing fish because your line is breaking then I have bad news for you, you need to learn how to tie a proper knot. Fish don't break 4 lb line.