Some themes are popular. They have been written about and examined by multiple authors. Sometimes an author returns to a theme multiple times, exploring it through different stories. And in some works, an author may weave multiple plots (some are sub-plots) within a single work. They can each reinforce the primary theme or raise questions about it.
The best thing the author can do is not to put many simultaneous subplots. Many secondary plots can often confuse the readers, and they will not understand the main plot.
For instance, in one plot "love conquers all", that is the love of two characters overcomes all the obstacles in their way. In the meantime, there may be a second, sub-plot, where two characters cannot over come the obstacles that come between them. Does that mean that their love was not true? or that love cannot always "conquer all"?
Nature versus Nurture
The relative importance of genetics (genes, heredity) or environment (care, stimulation, toxins). Books with related themes include
Love Conquers All
Love, generally romantic love, overcomes all obstacles put in the path of the lovers.
- Sense and Sensibility - in which love does not overcome all obstacles