You might consider the fact that Romeo is on the rebound from Roselyn...that he is fickle and incapable of real love at this stage in his life.
The thrill of the "forbidden" is also a possible reason. Both of them are made aware that the other is a family enemy, yet they continue with their plans. Not only is young love forbidden, but young love with a family enemy is most definitely forbidden.
Consider that arranged marriages were common in this time period. Juliet's arranged marriage to Paris is proof of this. It was rare that the arranged marriage included love between the two to be married. Perhaps it is possible to point out that these two were "arranging" marriage for themselves on an instinct (call it lust if you will) and were settling for that instead of true love.
The secret nature of their affair lends to lust and convenience. The nurse and the friar assist not because they believe the two to be in love, but for the possible outcomes of peace in the city between the two affluent and feuding families.
Consider defining "love." If nothing else, real love requires time; it takes time to get to know a person enought to value who they are in the heart and the mind. It is impossible to know that in the course of a day or two. Instincts can alert you to the possibility that love could develop, but lust can show up before a word is spoken.