If you were not already happy.
Controversial maybe, but I know from experience how often people attach their ability to be happy to their weight. But the number on the scale shouldn’t and won't dictate your feelings. I have been that person. I set a goal weight and I reached it. But once I reached it I wasn't happy, no I wanted more. This led me onto what could have been a slippery slope, constantly striving for more. But why do we think once we make it to a certain weight we will be happier?
Why won't I be happy?
One of the main reasons is that we attach meaning to our ideal weight. We see ourselves as being more worthy, more loveable and accepted once we are a certain weight but this isn't usually the case. These are links we make in our own head, I was still the same person once I lost the weight, I was just thinner. We need to give ourselves permission to feel how we want to feel at any weight. How do you think being at your ideal weight will make you feel? What problems do you think it will solve? Your weight is unlikely to determine these feelings or issues so take control now.
If you are unable to love yourself and your body as it is right now you will constantly be in battle with it. Body Shaming yourself into thinness is not a long term solution. You will continue to find things wrong with your body and this leads to the slippery slope I mentioned earlier. No weight is low enough, no amount of muscle is enough and so on. It is a vicious circle. When you start loving your body, and appreciating what it does for you it is far more likely to adapt and look the way you want. Start loving your body enough to nourish it properly. Exercise because you enjoy it, and it keeps you strong, fit and healthy. Then you'll start to see the difference.
Don't let the number on the scales define you. This doesn't mean you can't set goals, or work to improve but try not to attach your happiness to reaching that goal/weight.