Why do girls lose interest?

This has happened to me multiple times. I'm texting and messaging a girl for a few weeks and they seem really interested. They promptly text back even if I'm replying to their text hours later. They text me when I haven't texted them in a few days for no particular reason. We're flirting, and theres a lot of banter between us But without warning after a few weeks they just seem to lose interest. And ideas why? If it's any help, I'm also in HS so I'm dealing with HS girls.