you cant force people to play DnD if they're not into it. Having said that if they feel that there is too much grind maybe move away from combat, loot and leveling and try and get them to focus on roleplaying see if they get into their characters and let them have funny/stupid interactions. Dont take everything seriously and make the game not so formal. Also, depending on how old you are maybe have some drinks while you play and relax. Dont worry about the game just use it as a background for having fun with your friends, if they are having fun they will enjoy the game more.
This advice may not apply as I have no idea how your game is running. But when i played DnD last with friends most of us were just being very silly and it was a lot of fun, different people approach the game differently. If you want to be a lot more serious and focus on the mechanics of the game while everyone else just wants to fuck around and see what they can do then you might be playing with the wrong group of people. Basically you want everyone on the same page of how to approach the game.
Again this advice may not apply I'm just kinda guessing at what the problem is. It may also be that your friends just dont like DnD.