There are a number of sub-questions related to that overall question. But in general, someone worthy of a spot in your life should add value to your life, in more then one way.
Here are the other questions to consider though, when trying to make this decision:
Someone worthy of a place in your life....
1. Is someone with whom you generally enjoy spending time. Obviously everyone has occasional bad days, difficult times, or moodiness. But these are exceptions. They should not be the norm. People worthy of being in your life are those who you look forward to spending time with, and when you come away from time with them, you feel good.
2. Is someone you can trust. You can rely on them, know they will keep their promises and their commitments, that their words will be reflected in their actions towards you as well, and that they are not a person who lies. A person whom you can trust is someone you can believe in, feel safe with, and thus be close to.
3. Treats you with respect. This means many things, including treating you as someone with value, being honest with you, respecting your time, truly listening when you speak (without interruption or being distracted), talking to you in such a way as with kindness and love, their actions making it clear that their relationship with you is a priority, and they act in ways that show compromise and understanding within their relationship with you.
4. They are a good influence on you. They are someone who encourages you to be your best self. People who are good for you make you yearn to be a better version of yourself. People who are a good influence do not encourage you to lie, cheat, abuse alcohol or drugs, harm yourself, make choices out of fear or laziness, make choices based on their selfish wants instead of what they know is actually best for you and your happiness, etc. Great people want, for you, what is best for you (not what is best for them), and they bring out the best within you.
5. They are equally as interested in a connection with you, as you are with them. And their actions consistently back this up. There is generally equal effort to maintain the relationship. Equal sacrifice. Equal giving and receiving. Equal prioritizing of time. Equal gestures of appreciation and love. Of course, there are times when it will be somewhat imbalanced (if someone is going through a big life challenge, difficulty or change for instance), but the point is that overall, it should basically balance out and there should be clearly equal effort and interest from both ends.
6. They encourage growth within you, as well as are open to growth and learning themselves. Both of you are truly receptive and open to learning both with and from one another. Great relationships enrich both of the people within them in different ways.
7. Inspire you in some way. Maybe you are inspired by a friends sense of daring and adventure, or by their hard worth ethic, or the way they can create magic with their writing, or by their confidence and emotional strength, or their generous and deeply loving nature. You get my point. The list goes on. The most special, memorable and worthwhile people in our lives inspire us in some way, and make us want to be better versions of ourselves.
8. They seek to understand you. Who you are as a person, your dreams and fears, your passions, your ever-changing emotional landscape, your past, your feelings, your current life experience, etc. The people worthy of your company and heart are deeply interested in all that you are. They seek to learn as much as possible about you, to understand you as much as they can, and maintain this interest throughout their time knowing you.
And finally, good friends and people make you feel this... :-D