There are multiple instances in the strips that show Susie Derkins and Hobbesin love with each other, or at least attracted to each other. Here are some appearances of their "romance".
One time, Susie mailed a Valentine card to Calvin (to Calvin's horror and Hobbes' amusement) but it turned out to be for Hobbes. Hobbes was pleased when he learned the card was for him ("Me? Really? Hot dog! Smooch City, here I come!")
Susie also gave Hobbes an invitation to her birthday party. On the back it had a notice to "bring along that stupid kid that hangs around you, if you must."
When Calvin and Hobbes were having a water fight, and Susie joined in, Hobbes comically betrayed Calvin by giving Susie his water balloon, saying he is "easily seduced by a woman in a swimsuit", also saying he would do it again in a minute.
During a G.R.O.S.S. plan, Susie managed to kidnap Hobbes. Hobbes later admitted that she rubbed his tummy and implied that he enjoyed it.