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?
Depends on the context. Did you say something worth me responding to? If so, I'll respond. If we were talking and I lost interest, I wouldn't just not text you, I'd text you letting you know that so you wouldn't bother wasting your time texting me and you could move on.
Nope it just means I'm busy I will text them back later.
No not really BUT if he is giving you one two word shitty texts then thats not a good sign
Depends on situation. If you've done something that upset me, I'd still reply but I wouldn't be flirty or enthusiastic. If you're playing games, disrespected me, or did something that I considered to be a massive red flag/deal breaker then I wouldn't respond or have any interaction with you whatsoever unless there was a sincere apology, and VERY STRONG effort to correct the situation.