I think we set out to be. However, we all naturally will lean towards people with behaviours that appeal to us. My youngest who is seven has a personality which to me I find very interesting whilst my oldest who is nine tends to do stuff which is very annoying. The youngest also is naturally tactile always coming to me for hugs whilst his brother is stand offish and will only hold your hand for security.
Due to this I do tend to find myself leaning more towards my youngest. However, I cannot treat them differently otherwise I am neglecting the attention of one child to the preference of another. This cannot happen so I take time out to make my eldest feel special when his younger brother is not around.
So do I love one more than the other? Yes and No. Love is not just about where your natural inclinations are but how you deliver your care for them. My eldest can with my attention help develop our relationship better over time. This may change meaning I may prefer his company at some point.
Which leads to the next aspect can we all treat our own children equally? No, as our love in the sense of care has to change to appropriate the attention that child needs. If a child falls ill - we need to give it more love. When a child has more independence it may not require us as parents to love them so much.
Our hearts however, how difficult it may be, have to give love equally and to do this we may need to give our care (love) more to balance that difference.