Why is my 7 month old fm dachshound humping.

Your dacshund is maturing. You don't say what sex your dog is but given the age I would put it all down to hormones.

It could be that the hormones are causing the dog to realize that it can mate things and instinct is taking over.
Or it could be that the hormones are causing the dog to want to become more dominant and humping is its way of asserting itself.

Either way desexing now will really help the problem.

I'm guessing "fm" means female..

Females will hump. Its more of a dominance thing than a sexual thing. She is also learning who she is, what she is allowed to do and what she isn't allowed to do. And sometimes it won't end until she's been told to by other dogs, or if she is humping you, until you tell her to stop.

She is beyond the age of ready to be spayed. Spaying will reduce a lot of bad behaviors associated with hormones, not to mention, you'll prevent a lot of nasty cancers from forming.