This is because you're starting to place more emphasis on guys.
Yes, your friends may be dating. And you may 'think' these guys are decent, but who know for sure? And while you may feel you're somehow missing out by not getting noticed by guys, all that will do is tempt you to date and get caught up in all of that. And that will lead to break ups, heartbreak, bitterness. All your friends may seem happy now, but when they've had their heart broken a few times how happy will they be? When (and almost certainly some will) they give in and have sex how will they feel?
Often times people who are not dating are so focused on what they're 'missing' they forget there's a lot of bad that they are avoiding. Much worse stuff than what they're 'missing out on'.
And though you may not have a lot of guys chasing you, be glad. Because one day when a guy does notice you, he'll notice YOU, not some image you present. When i was 17 i met a girl who was your age. We spent many months becoming good friends. We were very close. She didn't do much with he hair. Didn't dress up (decent but not to get attention), she didn't wear makeup. She was a little chunky but still cute. She had acne problems. She was quiet. And i thought she was amazing. She didn't need to do anything for me to fall in love except be who she was. I encourage you to do the same, just be yourself, be content that you don't know guys approval. Enjoy your life, your talents and your time.