When you desire something and you ask the universe to bring you what you desire, there is a lot of space left in the middle of things for that desire to not come into existence. It's like asking a friend to stop by for a visit the next time they are in your hood. Your friend might stop by, but the timing might not work out. Maybe they don't really want to spend time with you one day. Perhaps they are busy, or they have other important things that they need to do. There is a lot of room for the meeting to never take place.
This is similar to when you ask the universe to bring you something, you leave a lot of room for it to NOT happen. You might claim the timing isn't right, maybe you don't deserve it, maybe you haven't been a good girl. The excuses can be endless, as to why you haven't received what you are asking for. You are asking a source outside of yourself to bring you what you desire, based on a timing that is outside of your power.
When you declare what you are creating, it's a whole different story. You partner with the universe, and decide to co-create what you desire. You are an active participant in making your desires into reality. It's like when you call up your friend, and you say, lets get together for tea this Saturday at 2:00. You are making the plan, you are setting creation in action by declaring your desires. You are not leaving room for a zillion unknown variables to interrupt your desires.
Your declarations are powerful. When you declare it to be so, then it is. Just as when you declare that there is a lot of space and room for things not to happen, then that is so as well.
When you are declaring what you are creating, you are seeing yourself as being part of and not separate from source [god, goddess, universe, whatever word you use]. You are seeing that together you create from the desires that are in your heart. You are working as ONE with an unlimited energy, and working towards a common goal.
Desires are the seeds of all of creation. If your heart has a desire, the purpose for that desire is for it to be birthed into this world. The seeds you plant and the desires you create into this world are your own unique way of sharing your essence with the world.