For Jefferson, the basic elements of the argument for separation lie in a lack of respect. Jefferson frames the argument that the violation of economic and political rights that the Colonists have had to suffer were the result of a lack of respect. Jefferson is emphatic in how the British government has refused to "assent" to the basic principles of dignity and decency that could enable a successful relationship and partnership to develop. For Jefferson, the critical argument that makes rebellion and dissolution absolute is that this lack of respect will not depart. As long as this imbalance exists, the Colonists will be treated in a secondary manner, denying them their full voice and activation of their rights. It is in this light where I think Jefferson's key arguments lie. In the second section regarding the "Grievances" that the Colonists have borne, Jefferson outlines all that has been done. In doing so, he solidifies his initial argument that separation from Great Britain is the only path that the Colonists can pursue. The relationship between both nations has been so degraded by a lack of respect that there is no other option.
For more background on the crafting of the Declaration of Independence, check out this video:
If you take a look at the Declaration of Independence, the middle section is a list of grievances that Jefferson writes Great Britain has committed. Jefferson leads into this by talking about what rights people have (such as life, liberty, and the pursuit of happiness), and that the purpose of Government is to protect these rights. Jefferson states that the King of Great Britain has continuously failed to do so, and therefore, it is the job of the citizens to replace the ruler.
A few of the offenses Jefferson charges the monarchy with are:
"He has refused his Assent to Laws, the most wholesome and necessary for the public good."
"He has forbidden his Governors to pass Laws of immediate and pressing importance, unless suspended in their operation till his Assent should be obtained; and when so suspended, he has utterly neglected to attend to them."
"He has refused to pass other Laws for the accommodation of large districts of people, unless those people would relinquish the right of Representation in the Legislature, a right inestimable to them and formidable to tyrants only."
"He has called together legislative bodies at places unusual, uncomfortable, and distant from the depository of their public Records, for the sole purpose of fatiguing them into compliance with his measures."
Those lines, and more, all are related to the main idea that the British Crown is not doing what is best for the people of the colonies and is failing to give them a voice. Thus, the colonists must act.