The main themes are
1) self - interest and preservation versus love:
Shylock's most famous words, "O my ducats! O, my daughter!" (II. viii. 15) are an indication that Shylock values his money (ie his ducats) as much as his daughter which would suggest that his greed and therefore his preoccupation with himself outweighs his love for his daughter.
In contrast, Shylock is very hurt and upset when he discovers ( III, i) that his daughter had so little regard for a ring that was sentimental to him and she sold it for something as flippant as a monkey! The money does not seem so important to him anymore.
This apparent contradiction gives more believability to Shylock's character as many people (whether in Shakespeare's day or now) are conflicted by love and money!
Continuing this theme, initially Bassonio only wants to marry Portia because she is wealthy and goes to great pains to ensure he will be a fitting suitor so that his mounting debt problems will be over.
2) Mercy and its divine quality (The divine quality of mercy):
Shylock is a stereotypical Jew of the 17th century and he is dealing with stereotypical Christians of the time. The 'Jewish way' -according to the Old Testament teaching- means that Shylock is harsh and appears unforgiving. If he is wronged (or thinks he is wronged) he wants justice or revenge. Mercy is a quality he is unfamiliar with!
The Christians then are the so-called compassionate ones. it seems, though, that their compassion is self-serving (the themes will overlap each other regularly). Shylock is not expected to exercise mercy but the Christians are because it is the 'Christian way.' Portia utters a well-known speech (IV. i.179) which would have been very familiar to the audience of the day and which they would have related to. you can just imagine the audience shaking its head at Shylock and his characteristic cruelty.
However, the Christian's level of mercy is not always beyond reproach as Portia, although she exercises SOME compassion is still guarded and forces Shylock to beg for mercy. Antonio also takes mercy to a point but to take away the religion of a Jew would be far greater punishment than taking away some of his wordly goods (even for someone like Shylock who values possessions so highly). So Antonio's compassion is questionable.
Mercy, in its purest form, is a completely selfless act - definitely NOT what we see in Merchant of Venice.
This can be coupled with Mercy or dealt with separately. It shows the nature of things and how life itself goes in circles.
Right from the beginning,Shylock blames the Christians for making him as cynical as he is. He claims to have suffered abuse and - being a Jew - he must get revenge when he is wronged. Forgiveness is out of the question.In (III. i. 60-61) when Shylock attempts to justify the "pound of flesh" (NOT a figurative pound of flesh to him) he directly accuses the Christians of years of abuse, "The villainy you teach me..."
It seems that much of the play revolves around characters who feel hurt, insulted, usurped and they therefore have to hurt back!
Although Antonio does show hatred towards Shylock (even spitting at him which is highly insulting) and being able to find justice (for himself) in forcing Shylock to convert to Christianity, he does show restraint eventually and , even when pressurised by other sources, he does not allow the cycle to perpetuate itself, drawing the plot to its conclusion.