The stability of 2,3-dimethyl but-2-ene is more than 2-butene. This can best be explained in terms of

More $\alpha$ -hydrogens are present in 2,3-dimenthyl but-2-ene.Hyperconjugation effect decides the stability of free radicals, carbocations and alkenes, Hyperconjugation effect depends on the number of alpha hydrogen present on the alpha carbon with reference to the benzene ring or the radical/C=C/positive or the carbon chain. More the hydrogens more the hyperconjugation forms. It involves the overlap of  sigma with p-orbital overlap.