As with all instances of pokémon aggression, it’s impossible to answer definitively without knowing the full context. The best thing to do for the time being is to take into consideration the surrounding circumstances and work from there. Is this recent behavior, or did it start when either of your pokémon (or both) evolved? Does your lucario seem to lash out only when your lycanroc does something, or does he attack on sight? If the former, what are your pokémon doing just before your lucario attacks?
Once you figure out what might be triggering your lucario’s attacks, then you’ll be able to work out a solution by figuring out what it is, specifically, that your lucario wants.
Until then, it may be helpful to you to browse our team building tag (http://bills-pokedex.tumblr.com/tagged/team-building ), as many others have encountered situations in which their pokémon have become aggressive towards their own teammates. Perhaps one of their situations may shed some light on your own.
Best of luck, anonymous!
The Pokédex According to Bill 