We had a dog years ago & she did the same,I wish we had stopped her when she was young as she ended up wanting to hump on my husbands leg nearly every night never anyone elses just his,it became a real habit & was very embarassing when people came to visit.
It didn't bother my husband he said she was doing no harm & let her carry on.
She had her daily hump until she died at 14 yrs old,so once they start they never stop.