There is nothing called straight and curved, it's all in the head :)
Lines are the best thing that can happen to an artist. Highly elemental, symbolic and magical thats how I define lines. In all my diaries I have a million lines everywhere; lines of different thickness, intensity & colors. People often ask me why I draw lines, I could never answer this question in fact it isn't a question. While I was in Madhubani, Bihar last year I met a beautiful person called Santosh Kumar Das, an artist and a warm human being.
Santoshji, a graduate from the MS University of Fine arts Vadodra is someone who uses lot of lines in his work. I stayed there for a month and spent a lot of time with him. Santoshji always said, "Observe each and every line when you draw, draw it from your heart. It is absolutely fine if the line isn't straight as long as its intuitive."
I came back with lot of energy in me and I started using even more lines in my work. I like them, I relate to them. They are very nice to you if you are nice to them. If you haven't found the joy in drawing lines from your heart, you are missing on a lot, trust me.
So if you have read this post open your sketch book and draw lines. Remember there is nothing called straight or curved, its all in the head. :) Till you get your sketch book you can stare at the above, its fun.