Most Helpful Guy
There's no guarantee it arrived at all - text messages aren't totally reliable, though they usually get there in the end - and it's far too easy to miss the notification, particularly if you get two close together. I've had emails from close family and not replied at the time because I got distracted: I certainly don't want to "break up" with my family members (well... probably not...), I was just busy doing something else and forgot about it!
Plus, if I *am* seriously interested, I'll be nervous about replying. What if I say the wrong thing, seem too eager, or put her off somehow? She signed it with XX... do I do the same? If I don't, will she think that's a bad sign?