It's natural to feel jealous at times but I rarely feel it. I am however competitive, which answers your other question. I saw a woman at a wedding once many years ago and she was so pretty and thin. At the time, I was very heavy. Maybe that's when I felt a little jealousy towards her and wished I was in that great shape, so the competitive part of me kicked in and I started exercising and lost a ton of weight. I never saw her again but by that point, I wasn't thinking about her. All that energy I was feeling was transferred into a positive one for me.
As for relationships, it will take a lot to make me jealous. I am very confident in who I am and it took me a long time to get here. I would tell anyone who feels jealous about anything to ask themselves why they are feeling that way. Do they feel inadequate? Or is there self esteem suffering? If so, there is nothing preventing them from feeling better. Whatever negative feelings they have, can be solved within themselves with work and dedication.