Saying "I love you" for the first time is not one of the milestones you'll read about in parenting books or be asked about by your pediatrician. Unlike other developmental markers, like rolling over, walking, or talking, saying "I love you" can really come at any time once a child is verbal. My daughter let me know when she was 10 months that she loved me. My son made me sweat it until he was almost 2. No matter when it happens, there are things every mom thinks when her kid says "I love you" for the first time.
Let's be clear: the first time your child says "I love you" isn't the first time they decide they love you. You're the parent, and they're a child, so they're pretty much genetically engineered to love you and vice versa. If you keep up your end of the deal (what with the feeding and nurturing and loving them back), things are going to be awesome for you. Still, very small children (toddlers and below) really don't know how to put names to what they're feeling which make sense, considering emotions are confusing. I know grown-ass adults who still don't get it, including yours truly (half the time). I have a lot of feelings, you guys! They're hard to parse! Like, I cry over some internet video every single day of my life and most of the time I really don't know what, exactly, I'm crying about. I can't put a name to that emotion!
Now, imagine trying to formulate thoughts around the huge, all-encompassing concept of "love" only months after you've finally formulated your thoughts around the concept that you have toes. That's what little kids are up against, really. So, be patient if it doesn't happen right away. It will. Until it does, rest easy knowing that they love you dearly, even if they can't express it the way adults do. And, when it does happen, get ready for the following roller coaster of emotions...