In this first section, there is limited imagery—only the swearing/contract theme, which develops a sense of love as a legal contract, and one that Romeo insists would bind him/them.

However, if you continue on in this same section, you will see two further image sequences in conflict. One is fearful, comparing their love to lightning, which is brilliant, but which is gone by the time you know it is there. The other is hopeful, comparing their love to a " bud," which will ripen into beauty in its appropriate time.

In other words, Romeo promises stability; Juliet counters with fears of love vanishing quickly, and hopes of it growing organically.
Greg