Actually this is an obvious question but it's not what you might think. Let me explain it to you, I've been an opiate addict for a long time and tried many drugs. Drugs that are 'uppers' have the most 'obvious' euphoria. For example if you take adderall/coke/meth/speed/MDMA you will get this shining bright euphoria, self confidence, energy, and other drug-specific feelings (for meth like you are king or for MDMA like you love everyone). However, you owe these drugs back what they delivered to you. After a meth binge, or lots of MDMA use, or staying up all night on coke you will feel like shit. To an extent this aspect is similar to an alcoholic hangover.
On the other hand, for many people who experiment with heroin they are underwhelmed (not including IV usage, but most experimenters rarely ever IV first time). They just feel good, chill, happy, but they feel like this spooky drug 'heroin' hasn't delivered. They are just mellow. Oh obviously it has all been a lie they will think. Heroin isn't spooky, it's chill. It's not addictive like everyone else thinks. It doesn't make you do stupid shit or stay up all day and hallucinate like amphetamines or coke. It doesn't empty your serotonin like MDMA or give you a hangover like alcohol. People tend to just think oh, what a nice drug.
So the next day they wake up and everything is normal. No headache or shitty feeling--just a slight afterglow of that nice feeling. Oh it was cheap as well! It only cost $10 for a whole night of being high! I thought people said heroin was expensive? And then next weekend comes... There are all these drugs I could do but I liked heroin. It didn't 'fuck me up,' I could still think clearly. No hangover. No feeling like shit later. I still was awake. It just made me happy and content with life. Oh and it's only $10! Well, I should get some more for the whole weekend. This is great! I will use Heroin on the weekends now!
Now let's say this person works and has responsibilities. He knows he can't go into work drunk, or on MDMA, or high. So he doesn't. It's actually simple. But heroin... Well the user might actually find they do better work on heroin. Instead of being sad or grumpy or depressed with his job... he is just... happy. Mellow. Content. Everything is fine and the world is beautiful. It's raining, it's dark, I woke up at 5:30AM, I'm commuting in traffic. I would have had a headache, I would have been miserable, I would have wondered how my life took me to this point. This point I'm at right now. But no, no, everything is fine. Life is beautiful. The rain drops are just falling and in each one I see the reflection of every persons life around me. Humanity is beautiful. In this still frame shot of traffic on this crowded bus I just found love and peace. Heroin is a wonder drug. Heroin is better than everything else. Heroin makes me who I wish I was. Heroin makes life worth living. Heroin is better than everything else. Heroin builds up a tolerance fast. Heroin starts to cost more money. I need heroin to feel normal. I don't love anymore. Now I'm sick. I can't afford the heroin that I need. How did $10 used to get me high? Now I need $100. That guy that let me try a few lines the first time doesn't actually deal. Oh I need to find a real dealer? This guy is a felon and carries a gun--he can sell me the drug that lets me find love in the world. No this isn't working, I need to quit.
To answer your question, heroin feels nice. That's all, it just feels very nice. You can make the rest up for yourself. Attach your own half-truths to this drug that will show you the world and for a moment you will feel as clever as Faust.
