What does Romeo’s final speech reveal about his fears or suspicions in Romeo and Juliet?

When Romeo speaks to Juliet, whom he believes to be dead, he marvels at the way her cheeks and lips still are still tinted with pink.  He was expecting her to be quite pale, as corpses are, and he makes much of the fact that her beauty is "not [yet] conquered" by death (5.3.94).  However, after he looks at Tybalt's body, likely mentally comparing its appearance to hers, he again asks how it is possible for her to remain so "fair."  He muses,

Shall I believe
That unsubstantial death is amorous,
And that the lean, abhorred monster keeps 
Thee here in dark to be his paramour? (5.3.102-105)

In other words, he personifies death, suspecting that death is actually living being, and a lover, who keeps Juliet with him in her tomb so that she can be his mistress.  However, he does not like this idea at all and goes on to say that he will now remain with her in the tomb forever.