This does my head in. It's non-stop with my 3.5 yr old too. What I've realised though, is that while it's always there, it escalates when I'm not being very attentive to her. I think what she's actually trying to communicate is 'I want to have a conversation with you Mummy'. DD's speech is brilliant, but still not sophisticated enough that she can initiate the range of comments, questions etc that it takes to have a proper chat about something without me supporting the conversation from my end. If I manage to tune in and actually stop, get down to her level and have a good chat about whatever's important to her right now, the 'whys' ease up.... For a little while!