A lot of people are only happy for you when they are happy. If they don't have anyone special or if their relationship isn't working, they have a hard time being genuinely happy for you. Be careful who you share your happiness with. A lot of people will smile in your face, but can't wait until you are having a problem. Some will try to secretly cause problems between you and your significant other. It can depend on how it will affect their relationship with you, and if they are in a happy relationship. If you start spending lots of time with your new love, they may feel left out. Don't neglect friends and family for a new love. You should be able to give some time to friends. If they are unhappy in or out of a relationship, it may make them feel depressed or sad because they want happiness as well.
Sometimes, I believe that some people are genuine when they say they are happy when you find that special someone. Most times, you can tell on their faces when they aren't. You can tell who really cares/doesn't care about your happiness when you meet that special person. Your circle will become smaller. The people who really care for you will stay the same towards you, regardless of what you can and can no longer do with, or for them. The ones who don't will change on you when it's no longer about them. It's not up to anyone to be happy for me if I find a special person, it's up to ME to be happy. I don't want on approval stamps from anyone. When it comes down to it, your true friends will be happy for you finding true love. Haters? Not so much, but they can eat shards of glass.