This is one of the most common things you can find in a book, but it's realistic (unless your MC is a sociopath) and that's great. Relationships are found everywhere, if it's with animals or humans, there are always some nearby. One of the most common mistakes that a writer can make when they create a relationship is that they make things rushed between the characters. Okay, let make that clearer. Raise your hand if you would openly talk and flirt with you crush? I don't think many people raised their hands.
People are very awkward and embarrassed when they are around people that make their hormones do backflips. I for one, don't have a crush but I have had one, it was very awkward to be around that person.
Okay, so next is when you have a relationship between characters. They are slightly more comfortable and getting more as the days go by. Your characters are still shy but not as much as they used to be. When you get to the part in their relationship where they kiss (if they do/want to)you will have to make it expected for the reader. DO NOT BRING IT OUT OF THE BLUE! I have never kissed somebody without talking to them about it five seconds before we kiss. So have many people.
I think you get the picture of how you shouldn't rush it. A relationship is a very fragile thing in characters.
this is where i ramble; have fun