According to universalists, why should we obey God? If universal reconciliation is true- so that everybody will eventually come to know God because of his son Yeshua, and therefore nobody will have their flesh burned forever and ever- why should we obey God?
There are two things that most universalists generally agree with- they deny that “hell” lasts forever, and they deny that we have a free will. The sovereignty of God- mixed with his desire that all of mankind will know him- means mankind has no other choice but to eventually know him. So by all means, anybody could live in disobedience if their will should lead them down that path; but there is another force at work in us- our love for God.
Our love for God does not come from some magic free will substance. We love God because God prepared us with the ability to love, and “because he loved us first”.

”Beloved, let us love one another, for love is from God; and everyone who loves is born of God and knows God. The one who does not love does not know God, for God is love.” 1 John 4:7-8

The love God provides for us is much too wonderful to reject.  We see his plan of reconciliation, and we know that God is good. Therefore, whatever he says is good- and that means his law is good. There is no better way to live our life than to live it according to his word. I think all Christians, and even Judaism, can agree that the life God provides for us now is far superior to any other lifestyle; as the joy he brings to us is far superior to anything the world can provide.
Yeshua says:

”If ye love me, keep my commandments.” John 14:15

The “if” is not a stipulation; it’s simply the truth. If we love God, then we will understand the perfection of his commandments, and we will desire to obey them. God does not threaten us with physical torture to force us into obedience. He is most certainly to be feared, as a child ought to fear their father; however:

”There is no fear in love; but perfect love casts out fear, because fear involves punishment, and the one who fears is not perfected in love. We love, because He first loved us.” 1 John 4:18-19

Why should we obey God?
Because we love God. It is a real, uncontrollable, unconditional- yet very much explainable- love:

”O how love I thy law! it is my meditation all the day.
Thou through thy commandments hast made me wiser than mine enemies: for they are ever with me.
I have more understanding than all my teachers: for thy testimonies are my meditation.
I understand more than the ancients, because I keep thy precepts.
I have refrained my feet from every evil way, that I might keep thy word.
I have not departed from thy judgments: for thou hast taught me.
How sweet are thy words unto my taste! yea, sweeter than honey to my mouth!
Through thy precepts I get understanding: therefore I hate every false way.” Psalm 119:97-104