It's usually a sign of unresolved insecurities-and often creates a huge issue with insecurities growing-instead of shrinking. Lots of info on that if you do tag searches.
I highly suggest you both read some books. Love Without Limits by Deborah Anapol is a good one and for figuring out reasonable boundaries-and understanding that they need to be renegotiated at regular intervals (especially to get started) read Opening Up by... Tristan Taoromino? Spelling??
"Love As Thou Wilt"