Rabbeinu Yonah writes [שערי תשובה א-ב] that the sin of pushing off teshuva is very great. If you have what to do teshuva for [and who doesn't] - NOW is the time!! Chazal give a moshol to a person who was in jail and the inmates dug an escape tunnel. Everyone escaped escape for this shlemiel who remained. The officer in charge of the jail said to him - LOSER!!! ["קשה יום"] Why didn't you run away when he had the chance??!!!
What did he do wrong? He was being a good prisoner and followed the rules by not running away??!!
There are few places worse than jail. Inmates in jail suffer from depression and have a high suicide rate. It is a horrible place to be. When in such a situation - one doesn't look at rules. He wants out of there at ALL costs!!!
When a person sins he is in Gehenom!! His soul is stained as is all of reality due to his evil deed. If he appreciates this he wants out of this sin at all costs. If teshuva were an AVEIRAH people should want to do it just to cleanse themselves and existence from the horrible filth and stench of the sin. If he doesn't it betrays a lack of appreciation for what he did and for the severity of transgressing the word of Hashem. THAT is a sin.
A fortiori [קל וחומר] for teshuva which is a MITZVA!! So he can extricate himself from this horrible place and get extra credit for it. He moves [metaphorically] from a putrid, damp, hot, lonely jail cell to a GORGEOUS airconditioned apartment on Central Park West facing the park, with a whole staff of uniformed men ready and willing to follow his every beck and call. THAT is teshuva!!! How can one not take Hashem up on his super-super-duper generous offer??!!
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This Rabbeinu Yonah answers the famous question on the Rambam - Why does a בינוני have to do teshuva to be a Tzadik, just do one more mitzva? If he didn't do teshuva the sin is so great it outweighs all the mitzvos he would do. [ר' איצילה בלאזר]
