"Relationship love" is an illusion I think. It seems to be almost entirely transactional with a side benefit of having a partner so you are not lonely … that is if you can stand to be around that person every single day for the rest of your life. The reason relationship require "a lot of work" is because of the expectations that people have of the relationship. One person is always more involved that the other and will be needy or dependent on the attention of the other person and jealous/upset when they don't get it. So, relationships require a lot of attention, time energy and work BECAUSE … someone in the relationship requires a lot of attention, time and energy from the other person. Being alone sucks too. Typical of life … sometimes your only options are crap.
The details of the transactional bit should be self evident.